Spray a good penetrant in the lock mechanism (use PB Blaster) to try and free up the lock. It is also possible it is broken internally.
Spray it and let car sit overnight. If it still will not latch you may have to pull the door panel and remove the door latch.
You can replace it with a new one or find one at a junk yard.
Monday, March 24th, 2008 AT 4:36 AM