......
What's all this about taking a cable from the relays down the front of the car for the brakelights. Surely if I take a feed from the existing brake lights then that will be o.k as the negative/earth is common throughout?
I am an electrical lecturer and a qualified electrician, and I still get shocked by auto electrics. (pardon the pun). 
When you come to wire the 12N socket up, you'll find all the wires you need that GM generously provided in a 9 way socket in the boot behind the rear trim panel. If you use the black/yellow (brake lights) wire from here you'll find that your trailer brake lights will not work. This is bcause it's fed via a 'relay' up front that, if used, extends the brake light check to your trailer brake lights, the 'relay' was around £20 a few years ago. So, if you use the socket provided you either use the 'realy', short the relay base to feed the boot socket, or as you say, take a feed directly from a brake light itself. You'll also find that there are two blue/black (rear fog light) wires, this is because a GM loom is 8 wires & uses a micro switch in the 12N socket to turn your car's rear fogs off while towing, so make sure you get the blue/black that is the feed and not the return.

But a qualified spark would be able to work all that out on his own!
