my 1993 850 seems to have the same problem. the first thing i did to shut it up was pull the blue "relay", which i actually thought was just the speaker. my car has no security system, other than a sticker saying "i am a graduate student, therefore nothing in this car is worth more $10."
a few days after i pulled the relay, the battery went dead, i figured due to the fact two courtesy lights appeared to be constantly on (1) the red light on the drivers door itself (2) the white light at the feet of the driver. So i pulled the fuse for the courtesy lights, i have been driving it for about 4 months without that relay or the fuse, seemingly, without any adverse effects.
what i am curious about, is whats the best way to diagnose the door ajar sensor. i'd like to be able to plug that fuse for the courtesy lights back in, since it also apparently handles some of the current for the oil level warning light, which has been on ever since i pulled the fuse. (i check my oil with ever tank of gas, the level is not low).
1993 850 20 Valve 5 cyl : had 190k miles when i joined this forum : nor cal - so cal - long island. Now i live in NYC, its got 259k miles, and sadly, the only car I've ever owned is going to be scrapped. RIP Black Volvo. You will never be forgotten.