I worked as a technician at a Volvo dealership for 5 years. I couldn't for the life of me figure out why they omitted a switch for the antenna on 850 models when it seemed that every other model from those years came with them. I even noticed that '97 and '98 960 and S90s had them and the switches looked identical to the dash switches on the 850. Turns out they do fit and there is something that can be done! After using this site multiple times for my benifit, I feel I should give something back. What follows is the procedure I used to put a stock(Or at least appears that way) Antenna Down Switch in my '96 854 Turbo for about $20 dollars.
The first step is locating a stock switch, if that is important to you. You could simply wire any old switch in between the antenna signal and make it work by mounting it on a spare switch insert on the dash, but I feel like I want a stock look to my vehicle and to impress people when they realize that it didn't come from the factory like that. If you don't care and just want to wire in a switch there is a tutorial on this site that shows how to do it. If you prefer my type of crazy, continue reading.
Find a junkyard in your area. Preferably one that lets you pick your own parts. Locate a '95-'97 960 or a '98 S/V90. The junkyard I went to had 2 and only one still had the switch. To the immediate left of the steering column, you will see a switch on the dash that resembles an antenna(see pics below). Pry it out and remove the connector. Remove panels or whatever you have to and cut the wiring for the switch as far back as you can. You will thank yourself later. You should try to get around a half foot of extra wiring off the connector. At this time check for any 850s and pull off any parts you need too
. Yoink some LED bulbs from those dash switches too. They come in handy and are expensive at the dealership. The switch with the connector and wiring should be pretty cheap.
At this point, I got the wiring diagrams out. I checked the 960/S90 radio wiring and found out that it is more simple than you'd think to wire this thing in. The switch seems to be just a simple single throw type. It interrupts the signal sent to the antenna module telling it to keep the antenna up. It is also wired to the dimmer/rheostat so you can see it in the dark. Four wires, no problem.
Looking at the switch connector, we have a red wire in position 4, a red wire with a black stripe in position 3, a black wire in position 5 and a brown wire with a black stripe in position 2. Looking at the wiring diagram for the 960/S90 we can see that the red and red/black wire control the antenna signal. The brown/black wire is the positive rheostat splice and the black is a ground splice. All of this can be easily patched into the radio harness of an 850. If you notice any differences in wire colors or configurations, please reference the wiring diagram for the vehicle you removed the switch from. This tutorial's wire colors and positions only apply to the vehicle I removed my switch from and Volvo is known for switching things up randomly.
After you have acquired a switch and stock harness and are familiar with how it is wired, you should look for a good place to mount it. Since I didn't get quite enough extra wiring, I am going to wire mine as close to the radio as possible. Even though the 960/S90 had it mounted on the left side of the steering wheel, it might make the most sense to mount it in the dash position second from the right. You may have to reroute some existing switches depending on your options. My vehicle came with TRACS, Info, Sunroof, and heated mirrors. This required removing one blank panel and moving everybody to the left one space.
If you haven't already, remove the radio. This is very easy and is accomplished by pushing in the two tabs on the lower sides of the radio and pulling on them to release the radio from its housing. If you know the code, go ahead and remove all the connectors from the back of the radio. If you don't the code, a phone call to a Volvo dealership parts department will point you in the right direction. After removing the radio, the larger connector with 14 or so wires coming out of it is what your looking for. If you can, pull this harness away from the dash to get more room to attach the switch. There's a clip that's hard to see holding it in place. You're going to want to splice in about an inch away from where it snaps into the clip. Make sure you have enough space to connect everything back up when the switch is installed. Check out this wiring diagram (available on this site).
You can see the BL-W (Blue with white stripe) wire coming out of the radio at position 3A. This should be your signal wire to the antenna module. To test to make sure this is the correct wire for the antenna, plug in just that connector and turn the key to position 1. The radio will flash "CODE" and the antenna will start to extend. Cut that Blue wire with the White stripe where you intend to hook the switch inline. The antenna should be retracting back down once it looses it's signal. Disconnect the radio, remove the key and solder or butt connector the switch in line with the antenna signal wire. The Red wire from the switch should be connected to the Blue/white wire at the radio connector and the Red/Black should be connected to the Blue and White wire leading away from the radio back towards the antenna assembly.
Once you've secured the wires by means of shrink sleeve, electrical tape or some other means, go ahead and plug the connector back into the radio. Turn the key to position one and check that the antenna goes up and down with the switch when the radio is on. Congrats! Now for the fun part.
The Dimmer switch controls a large number of lights inside the car and varies voltage by means of a rheostat slide in the switch. All of these lights connected to the dimmer appear to be in a parallel circuit for power and ground. Were going to access that system too, since it can only improve the "stock" nature of this add on. If you do not wish to, you don't have to do this step. It wont effect the switch. You just won't be able to see it in the dark. I feel comfortable enough in my electrical ability so I'll take the plunge. To be absolutely sure we are accessing the right circuit, take your Voltmeter (Key ON, lights on) and check for voltage at Pin 10A(Pink wire) and 11A (Black wire). These appear on the wiring schematic as (Rheo+) and (Rheo-) respectively. With your DVOM hooked up check the voltage with the dimmer lights all the way down and all the way up. You should see the voltage output go from low (3V) to high (11V) depending on your battery's current state of charge.
Now that we're sure we have the right circuit, connect the Brown wire with the Black stripe (Switch side) to the Pink wire(10A). This will form a parallel circuit on the positive side of the rheostat. I would recommend against using Scotch Blocks but if you cant get a butt connector or soldering iron in there, they will probably work fine. Remember to give the harness for the switch enough room to be plugged in. Do the same with the ground side. Connect the Black wire from the switch to the Black wire(11A) from the radio connector. I ended up cutting the wires and using butt connectors. However, I had to go through a few because the size wasn't exactly right and they kept coming loose.
Once every thing is in place, hook up the switch and check that the bulb lights up. Mine didn't. Turns out those bulbs don't survive well out there in the junkyard. Thankfully though I stole a few extra ones and tried a few of those. Of the 4 I ripped off, only one worked. But it did light up with the rest of the switches and looks great. At this point, put everything back together and bask in the glory of a job well done. Your 850 now has the Antenna Down Switch that was standard on some Volvos around that time, but not yours. You have improved upon a minor shortcoming in the design of you vehicle and done so in a "stock" way for about $20 dollars and a few hours of time. No more dreading the car wash. Cheers!