The 97 GLT wagon that I bought 5 years ago has a bad combination. Black upholstery and no courtesy lights. I got it with about 100K on it and it now has 135K on it, with no issues. Except I never have had even a glimmer from the overhead courtesy lights, nor from the rear passenger courtesy lights above the doors.

Pulled the overhead panel and checked bulbs and switches and all are okay. Fuse in #15 slot is okay. Presently it is in the shop having axles replaced and I asked the mechanic to see if he could find the power default. He checked the overhead again and guessed it was the light relay under the fuses. I pulled that relay on my '96 T wagon to try on the '97 in his shop, but found that pulling it did NOT kill the overhead courtesy lights. I then pulled all the other relays at that location, one at a time, and none of them shut it off. So much for that experiment.

The small lights at the front of the front doors, I think they call the front door edge marker lights, DO come on when the doors are opened, indicating the door-frame switches are working. And the seat belt warning lights imbedded in the overhead console come on for 30 seconds or so when the car is started, so there is power there.

Only clue now to go on is a brief passage in the Haynes Manual:
'The courtesy light micro switches are incorporated in the door lock assembly together with the central locking motor. Replace entails unsoldering a number of small contacts and soldering in the new wires. As there is a risk of damage to the central locking motor and the lock itself, this work should be entrusted to a Volvo dealer."

Is this a lead worth chasing or are there other avenues to pursue. Indeed, on a working system, the lights come on inside when the vehicle is unlocked, and stay on for 30 seconds, same as when leaving. I could see a microswitch failure there preventing lights on when the car is unlocked, but one would think that there still would be power to the overhead courtesy light buttons. Wiring diagrams in the consumer manuals are chassis wiring and too skimpy to help.