Krishen, It will lock when central locking is used but then unlock instantly.
I think I got it figured out. I had to fit a new door handle since the last owner destroyed his when trying to repair it. The door handle was listed as for the Corrado but it needed quite a bit of fitting. It looked like the striker was putting too much tension on the opening latch. I had to grind down the striker to lessen the tension. It took a ton of trial and error but it finally worked. Hopefully I wont have any other problems with that door since I'm tired of taking it apart and putting it back together!
Thanks guys!