thats the prob with 90spec, its more fiddly to add accessories like this! pre-90 just has a load of spades on the back you can splice into, simples.
If you check out the FAQ there is a picture of the rear of the fusebox, D/9, D/11 means plug D, pin 9 or pin 11. As I say you'll probably have a spare black wire hanging off the D plug, check its ign live and if so splice into that! On mine what I did was go down the scrappy and chop a D plug out of a MK3 Golf, along with the wire from it to a jumper block above the fusebox in the dash somewhere.
These jumper blocks are well handy, you plug your supply wire in then you have 3 spare pins ready for plugging in accessories

andy the relay is switched from the ign live, so as to provide a good switched live feed to the motors. there arnt many good ign live sources with enough juice to run the windows direct, the blower fan switch is once exception but then you might have trouble running the windows if the blower was on as well!