Unusual Central locking problem.. 2001 X5

[shedtastic]

So I've only had the car about 18 months and there were numerous issues to fix, the central locking woes were way down the line..

But now that the cooling system is refreshed, the air springs replaced, the oil leaks are hopefully fixed and the aftermarket stereo is in, it's now made it to the top of the list.

I've searched on here, but the symptoms don't seem to match what I've read In other posts. In summary:

All the doors will centrally lock from the remote.
Only the fronts UNLOCK with the central locking.
The rears won't even unlock using the switch on the console.
I always have to reach in and unlock the rears by pulling on each rear interior door handle.
I can hear noises in the rear doors I think when I try and remote unlock them, I think.
Now even the passenger door is starting to not unlock with the driver's one, it tries, but struggles and you can see that the "pin" only pops up about 1/3 of the way. It's as if the mechanism is experiencing too much resistance, or not getting enough juice to get the job done. (But silicone spray has not helped!)

Any ideas?

Neil