Wow, wish I had been able to read this before I installed my Mk 1 way back when. Would have saved me one run of wiring, with the front-rear speaker trick. Ah well. Personally I have a 94 Golf, and the only other trick I might mention is this:
On the 94 model the glove box is where the air bag is on later models. I don't know how hard the air bag is to remove, but the glove box is a piece of cake. Removing it makes accessing everything required behind the dash much much easier. If you can, try it.