Sorry to rehash a very old thread but my added info may help someone else who was still stuck like I was.

I found the white wire worked to actuate all doors, but couldnt find a working solution after wiring in the relay like Fobz showed.

That relay set up didnt work for me because I had 4 alarm wire triggers - 2 positive triggers (lock/unlock) for a relay, and then 2 negative (lock/unlock) trigger outputs.

In my case I had to change pin 86 to an earth, then use a positive lock trigger from my alarm onto pin 85 to do the switching. Other side of relay was same as Fobz.
After a big headache, it works perfectly!!