The moulded serial cable built into the harness didn't have enough pairs in it, so the +12V lines on the serial never got hooked up. I wrote to the BBS quite a long time ago saying what the differences were between the early and later Mk2 harnesses were. I'll try and look up the post for you.

Found it! It's here. The +12V wire on the serial cable just comes off the Amp remote line anyway, so there is no difference between how it is on the later empegs and how you are doing it really.