Broken Locking Mechanism...

I own a 1991 Infinity G20 with a broken driver door locking mechanism. I cannot lock the driver door with the key, but I can unlock it. Sometimes I can lock the door by mashing the interior lock tab down, but not usually. The lock tab feels loose when in the unlocked position and really tight when I try and force it to the lock position. There are no funny electrical problems, I am quite sure it is a mechanical problem. Please let me know how it can be fixed or replaced. Thank you for your time. -ERiC