2005 Tacoma Driver Door Wont Lock

I just bought a used 2005 Tacoma. The remote locks did not come with it, so I have to use the key and push the button everytine I want to enter, exit the truck. When I get out of the truck and push the lock button, about %50 of the time all the doors lock and stay locked; and the other %50 of the time, all of the doors lock, and then the drivers door will unlock. When I go to manually push the lock, it will stay locked, but when I shut the door, it will automatically unlock itself. When this happens, I can not get this is door to stay locked. Anyone have any suggestions?

 

My 06 will lock with the keys in the ignition vehicle running. If there was a work around for the auto lock feature it may help both of us. Anybody?