The switch that operates the trunk light is located inside the trunk latch. Page 23-199 of the 1991 service manual sets out the latch test procedure (SM is available from a link in the Prime library) . Unfortunately, the test procedure only describes the trunk open test. You will need to place something in the latch to simulate the trunk closed position to test that the switch goes open. There are two switches in the latch, one for the light the other for the security system and both of them should be open when the latch is in the closed position.
Begs the question how you know the light is still on with the trunk lid closed.
Ah - that makes more sense. Clearly my morning caffeine dose had not kicked in yet!
That said, the same comments about testing the switch operation in the latch apply. The switch that operates the trunk light is the same switch that provides indication for the safety indicator in the dash. There is an easy first check to do. Just unplug the three pin connector that goes into the trunk latch. If the dash light goes out, the problem is the switch in the latch. If the dash light stays on then the problem is elsewhere and could be an indication of problems with the circuit board in the instrument cluster.
If the problem is not the latch switch, assess the status of the vehicle battery before tearing out the instrument cluster for inspection. Me, along with 3 or 4 other owners reported issues with erroneous indication on the safety check indicator (mine was the engine lid indicator). The problem was not caused by a faulty switch. Even though the car battery was not showing obvious signs of failure, in all these cases the problem was only resolved after the battery was replaced. I offer no logical explanation for why this resolved the problem.