Boffah
Your situation sounds like mine---the compressor works but rear won't pump up---in my case it was the relay. The two small diameter wires in the relay harness have to do with the LLS computer in the dash. The far right one is a ground---the other comes from the LLS computer---it's what trips the relay to activate the compressor. You have two choices---buy a new relay at the dealer for about $104---an easy install----or replace the LLS computer in the dash which I would bet is a lot more expensive---since on this site the relay fails often, I'd reco you get a new relay or used at bone yd and hope that fixes it---If it doesn't then it's probably the LLS computer in the dash---be SURE all the contacts in the relay are not corroded and that the far right small wire goes to a good ground.

If the relay harness connectors aren't corroded---then the odds are it's the relay itself.

Hope this helps