Just had to do the same on my '92 8v - Wife's mk3 came without a stereo but with an ISO block, however Halfords list gives 2 options and I now know it needed an adaptor loom as the +ve and switched are crossed over. Same as standard Mk2 (on my '90 Jetta). TOP TIP get down to Halfords NOW as they are clearing out obsolete adaptor looms - normally ?11 for a loose ISO block, how about one for a Daewoo for 60p - just chop off the end and hey presto a cheap ISO connector. If trying to fit an ISO block with just a clutch of loose wires, I don't think you can find a switched feed so do without or tap off the glovebox light, a rule of thumb is all the -ve have brown tracers, and simply look at the back of each speaker for the colour code. Dry run before soldering as you can't easily tell left from right though front and rear will be easy to spot. Better still go for solder and heat shrink sleeving and it will all fit back in without scraping your arm behind the dash.
I have gone for a simple CD tuner and nothing too loud, as the car has the classic digifant knackered idle and I need to listen for running on!