I have the exact _opposite_ problem you have... and I was just thinking the same thing this afternoon that you did, swap the passanger's lock for the drivers... i have trouble locking my driver's side door lock with all 3 of my keys - no problem unlocking though. My solution to both your problem and mine is this... leave the bad lock in the passenger side. I dunno about you but when I walk around my car to the passenger side I go 'round the back, which is a perfect opritunity to either lock/unlock the doors from the trunk lock.
Just a thought
Cheers for now
91 300E, with an "almost" broken lock