I've been suffering from this problem myself for a while now. My car is also a 93 850 (~150,000 mi). I've had it into the local dealer a few times (a mistake) and no progress has been made.

The only difference with my situation is that the rear driver's side door has been (mostly) locking and unlocking when I use the key on the driver's door or the trunk lock. But all the rest are totally dead.

Now, the dealer has replaced the relay and supposedly checked through all of the Volvo sevice manual checks and has not found a problem. We've tried resetting the fobs. I have even "borrowed" door lock assemblies from a yard to try replacing them all, also to no help.

So, I'll follow this thread closely and I hope somebody has some words of wisdom for this.