Hi. Delighted to meet you.
I've done a pretty thorough job hunting for information on the W140 Coupe... there's not much to be had from normal sources.
I don't think you'll get much assist from the Mercedes Club as far as documentation, etc. Nobody I have found sells manuals for these cars.
The three best sources have been this forum (there are several Coupe owners and excellent techs here), the ALLDATA subscription I have, and a set of original Mercedes Parts Microfiche for the W140C that I scored off of eBay. Between the three, I've managed to muddle through everything so far. The ALLDATA site allows you to download and print the pages you need for quick reference later... really handy when checking electrical schematics.
Lest I forget, there's alot of info on JimF's site. (Sorry Jim!)
You'll also find Phil of the Partsshop related to this site as very knowledgeable and willing to find parts for you at fair prices. The good news is that most of the powertrain and suspension parts for your car are the same as the big sedans. There are lots of those, so used parts can be had. The bad news is that anything unique to the Coupe is usually hard to find... and expensive. Try pricing a headlight! The Coupes are pretty rare, there aren't many in wrecking yards. Heaven help you if you need a hood. None of the body parts will interchange with a sedan.
I haven't had any issues with my seat belt extenders; they work fine. Don't think its a terribly complex system so shouldn't be too awful to troubleshoot. Sounds like the sensor that tells the unit the car door has opened/closed is intermittent.
Anyway, let us know how you make out. I've been pretty much all through my car doing standard stuff... so, if I can help, post here or write me at email@example.com
Not many of us W140 Coupe owners... we gotta stick together!