I recently installed a "wing" on my 05' V70 and since I had trouble finding good instructions online I thought I take some notes and share what I learned from doing this job.
There is a mounting kit available from Volvo for about $58 which includes all the hardware needed, plus the templates for drilling the holes from the outside. There are also holes to be drilled from the inside, but they are clearly marked.
Part number for the Kit: 30664143
Unfortunately, the instructions that come with this kit are a bit confusing and in parts even incorrect. More on that later.
Also, the screws supplied with this kit are the wrong length. Volvo's one size fits all does not work in this case, but again, I will explain at the appropriate step in the process.
This is the spoiler I installed, a good used OEM volvo part off ebay, even in the correct color for my car:
- 1/8" drill bit
- 3/8" drill bit
- 5/8" hole saw
- 10mm wrench of some kind, anything will work
- power drill (I use a cordless drill for almost everything I do)
- torx 25 driver
I found that the 5/8" hole saws are hard to find in stores. After wasting some time looking at the local hardware store, Home Depot and Lowes, I resorted to using a wood drill. This was VERY tricky and I would strongly advise against doing that.
After the fact I found them on Amazon for around $10. Do yourself a favor and order one.
First step is removing the tailgate panel and window surround trim, which is easy. The panel is held in place with four (4) t25 screws. Remove those and then pop the panel off.
Once that's off you can pop off the window trim. It slides down and out.
Next step is to apply the templates to the car and securing them with some tape. I then highlighted the hole locations with a marker, otherwise they are kind of hard to see.
There are two (2) layers of material, so I will call the outside layer "top" and the inside layer "bottom". The following image shows the design of the tailgate with the layers:
- spoiler-installation-layers.jpg (41.56 KiB) Viewed 9604 times
I am showing only the driver side, but the passenger side is the same.
Bottom holes. Note that these are marked already from the factory. I would advise against drilling the center hole. This is the hole location for the harness pass through, but it is in the wrong place. The instructions give dimensions for this to be drilled from the top, but they are also incorrect. Found out the hard way.
What I would do next time is to transfer the holes from the pads by laying them on the outside of the tailgate after lining them up with the mounting holes. I don't have a photo for this, but maybe this image helps:
- Spoiler Gasket.JPG (22.81 KiB) Viewed 9617 times
Now its time to prep the spoiler for installation. The connector needs to be undone, and the pins need to be pulled out. It should look like this:
Now comes the big moment. Line up the spoiler with the holes and stick it on. As this is done, the brake light harness needs to be fed through the clearance hole.
After that the hardware can be installed. Like I said earlier, the provided hardware did not work out for this, see the following photo for the correct sizes. You will want to be careful here and not overtighten.
Also, I choose to remove the old brake light. You don't have to do this, but if you want to, there is a little pin in the center that has to be removed and then it can be slid sideways and out.
The kit comes with a black decal to cover up the old brake light. Mine was all crumpled and therefore I did not install it. Honestly, this doesn't bother me. You can't really see it anyway behind the spoiler.
If you have done this and have suggestions for improvements, please post them here.
Dirk
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One question though: how did you remove the pins from the connector?! 
I can't just pull them out. The pins are stuck and I don't want to destroy the connector...