Old Timmer trick or what?

[67chevemall]

Well I thought is was going crazy.
But after sleeping on it I can't figure it
out. First of all the truck came with no
door keys, so in trying to replace the cylinder
I found the doors will not lock at all.
If you lock the door and slam it the lock pops up and open. If you shut the door with the window down and try to lock the door it will not lock. Impossible to push down on the knob.?...?
Any ideas. My guess is someone got locked out to many times and made a fix....