I don't have the cable loose, but just saw it recently. It's a small black plastic block that clips into the square hole, and is drilled at a 30 degree +/- angle to illuminate the ashtray when open.
Ah. Ok. I think I've seen one before. I just didn't realize what it was for. I got the lamp socket end and the "wire" the other day. Now I'll have to check a couple of ashtrays I have to see if one of them has it. Thanks.
Man, I know I have a spare ashtay somewhere. But you think I could find it? Uh, no. Oh well, a new quest. Like I need an excuse to go to more swap meets.