Just yesterday, I took my front passenger's door panel off and pulled the lock assembly. That door has been giving me occasional problems since I bought the car used in February of 2000. Fortunately, it only happens very rarely, but I've been having the same problems as you from that one door lock, and only that one.

I know it's that lock, because the front passenger's foot well light illuminates whenever the door switch doesn't activate. (That might help you determine which door is causing your problems.) Usually, openning and shutting the door again solves the problem, but it was getting worse to the point that I couldn't shut the footwell light off with one simple slam of the door. So I openned up the door to take a look around.

Removing the lock assembly involved pulling the interior door panel off, removing a small panel that conceals the lock assembly, removing the outer door handle (two bolts) and interior lock button, and finally the lock assembly (also two bolts). It took me an hour to figure it out, but will probably take about 15 minutes the next time. If you're not fairly competent with pulling things apart and getting them back together again, you might want to leave this one to the dealer. Any more than an hour's worth of labor charges should be declined or argued.

The microswitch is located on the bottom of the lock asembly. The assembly is in a molded case which it doesn't appear to be possible to disassemble. I'd recommend replacing the entire unit rather than trying to trying to pull it apart.

My problem is rare enough that I probably won't fix it, but if I ever do, I'll probably go for a used part. I don't know, at this time, what other years/models' locks are compatible. I'm sure a new part is going to be pricey.

Hope that helps.

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James Rothe, '99 S70 T5M, http://www.monmouth.com/~rothe/carstuff.html#Volvo