2001 Park Avenue base model. Starting five or six months ago left rear door would not unlock using key fob. Could usually get it to unlock and open by flipping inside lock actuator a few times. Got to point that it would not unlock/open using that method. Took all of rear seat out and removed door panel and was able to get door to open. Tried opening and closing door a few times and sometimes latch would not engage and keep the door shut. Obviously, I need a new latch assembly, but now cannot get door to unlock/open. Is there any over-ride to unlock the mechanism?

Sounds like the latch assembly was rusted up...........when you had the door panel off, should have soaked the latch assembly from the inside as well as the outside(moving latch) with PB Blaster.......operate the latch several times, and then spray again....do this several times....

When you use the power door switch or remote.....does the actuator move the manual door lock slider(can you see it move the slider from lock to open and then open to lock?

About all you can do is move the rod that releases the latch, repeatedly and push from the inside....careful it might release unexpectedly.....

I'm riding around with no back seat or door panel, so no problem to soak latch mechanism with lubricant.

I unplugged wires to lock actuator, so I'll have to re-attach and test. I'll post the results later

TECH II, you certainly nailed the problem and solution.
I was able to get a can of WD-40 inside the door and blast some of the latch mechanism and open the door. I blasted the rear parts of the latch from the outside of the door and the latch now works when using the fob or the inside ****. Should I repeat the process with a more "permanent" type of lubricant other than WD-40? The WD-40 seems to evaporate pretty quickly.

Use a graphite spray lubricant and you should be good.

My all time favorite is PB Blaster....

You might want to see what you can do to get the WD-40 cleaned out as best as you can first. It attracts dust.

I use lithium on door latches.