no 90 isnt late enough, talking late 91/92 models.
you can add the key-in-ign live easily though, find a mk3 or passat or something in the scrappy and look at the ign switch pin S or SU for the brown/red wire. follow this back to a square 1 pin plug, chop that then remove the wire from the ign switch using a thin screwdriver to release the locking tab. then simply pop it back into your switch in the right spot, splice the wire to the right pin on your stereo plug
if you need the pin for the stereo plug you could steal that from the donor car as well, or chop the yel/white or blue/grey wire off your mk2 plug, then pop it out and move it to the ign live spot once you splice it on the end of the brown/red wire. the pins in the stereo plug are the same as loads of the plugs in the bay and fusebox on the later cars too, so endless places you can pinch one from
blue/white is lights on feed, and yel/white is speedo signal, neither of which are required on modern stereos.
if you're as anal as i am, you could strip the entire red/brown wire out of the donor car without cutting it then fit it to your car, but you have to have a fair bit of wiring OCD for that
on the passat/mk3 the wire runs to plug H1 pin 7 before going off to the stereo, so that pin will need popping out too. (see my fusebox faq for more info on fusebox pins)
