You would need to remove the door trims to have a look, but be careful when removing the trims because you can easily damage them, just gently pry the cards off once you remove the window crank, arm rests etc..The lock is activated with a lever and there are plastic nubs that slip over the metal levers in the door. If the plastic has broken off it won't active the lock. Of course the locks could be stubborn from years of use and neglect also.
The good thing is it's all pretty simple once you have access to it, and you can buy these parts relatively easily if needed.