I just checked my car:
Key turned to position II: warning chime sounds, light on for about five seconds, goes off with the warning chime. It goes on and off with the one on the dashboard.

I believe the seat belt lights are operated by the chime unit, under the steering column. This is blue box, slightly larger than a relay, with slots cut into it for the chime. If the chime is working, I'd check out the bulb.

You can access the lamp from the rear seat:
Remove the rear ashtray. Now look at the ashtray insert in the console: there are two plastic tab-like things at the bottom of the insert that hook it to the console unit. Lift these up with 2 small screwdrivers and remove the insert. There are "bumps" molded into the top of the insert, so the whole thing kinda swings out from the bottom. I can take pictures of this process later if you need them. Anyway, now you can reach up inside there and slide out the bulb holder. Grab the end where the wires attach and slide it towards the front of the car; the red lens and reflector remain in the console.

The way I described is more complicated than just popping out the lamp ass'y, but I get kinda nervous snapping things in and out of the old brittle plastic, and I don't want to get gouge marks from a screwdriver onto the back of the console. Also, you could remove the junk box and go in that way. I prefer going in through the ashtray cause the coin boxes fall apart easily too.

I would check out the bulb first before tearing into the dashboard. Hope this helps and let me know if you need pictures.

Drew
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formerly non-registered at drew b.
green 1993 240, 164k 'Cream Puff V,' formerly: blue 1988 240 DL 'Cream Puff III'