How To Open A Deadbolted Door: ( Procedure courtesy of Russell Jones )
Just had a successful weekend regarding the locking on my 89 525 - where the rear door had deadlocked itself and wouldn't undo, so I was stuck with a door I couldn't open. As there were not too many ideas on this issue I thought I'd post the solution, in case anyone else has the same problem sometime.

1. Take out the base of the back seat
2. Peel off the door seal from the inside and remove plastic trim fixed to the floor (held in by large trim clips)
3. Undo the 3 screws in the trim (1 in handle, 1 behind door operating handle and 1 behind ashtray) & unscrew lock pin.
4. Pull the trim off at the top by the chrome trim, then using a long thin rod, pry out as many clips as you can down each side of the door.
5. Using a large screwdriver, push the cable end out of the door operating handle.
6. Open the window, and leaning in, grab the handle in one hand and the armrest bit in the other,, and pull the trim up and out. This will be a bit stiff but it will go eventually.
6. With the trim off, disconnect the window switch and take trim out of the car (wahey!)
7. You can now take out the offending lock solenoid using a 6mm ring spanner, and unlock the door.
My lock seemed to have corroded pins on the connection, so I cleaned these up and although I thought the solenoid was knackered, thankfully it seems to be working ok now.

If you leave the solenoid disconnected (as I was going to do until I got a new one) the deadlocking and the alarm will not work. What I was going to do if it was dead, was connect the solenoid back up, but not connect it to the locking mechanism - and just mount it as far away as the drilled fixing holes will allow.