Well, I have a 1991 Caprice Classic with the twilight sentinel option and while I have not messed with it much, I think mine is broken too. I could not get it to work at all if I was doing it right. Kinda sad, because this was a big reason I chose this car over a 1990 Lincoln Town Car that didnt have the option. I just assumed on the Caprice that it worked... I didnt know it was an item that was well known to break down.
Where is this module on a 1991 Caprice... steering column or kick panel? Kick panel sounds easier for sure, but knowing my luck, its in a hard spot to get to.