The type 89 Audi is so simple to add a cupholder to it almost seems foolish to describe... You just buy these two parts, for about $10 each, give or take, and snap them into place.
7DN = "Platinum" (grey), as pictured below. Other colors are probably available.
On a visit to my local Audi dealer I was indulging my fantasies by "test-sitting" a 2001 TT convertible (and making vroom vroom vroom noises). Of course I have to touch everything and examine all the parts Audi has come up with that might fit my Coupe. Then I notice the cupholder - which is an elegant little affair mounted on the center tunnel, all brushed metal and good looking black plastic - and not weirdly made to only fit that car! It is capable of being bolted with its normal mounting technique to any horizontal surface. So, while I am picking up my parts, I figure I'll check out how many hundreds of dollars it costs so I can forget about it. It turns out to cost only about $20! The part number is 8N0 862 531 A 6PS, in black. I think it is available in several colors for the various TT interior schemes.
I am still not sure how I will use this baby, a tool box location seems likely, though.