i am a mechanic and this is one of the most tricky things to do depending on the problem. all you can do is move the seat as far out of the way as you can, get as many retainer screws as you can see out, and try to gain enough room to get your hand between the door panel and the door, and try to get on a lock rod or the door latch itself. make sure the door is unlocked even if the rod looks like it is. it takes LOTS of patience, but you can get it if you take your time. best of luck.
to verify if the sensor is what's causing the problem, disconnect the connecter from the coolant temp sensor, and jump a paper clip across the terminals. turn the vehicle on after doing this, and see if the gauge has moved in the dash at all. try the same thing with the connecter disconnected. if the gauge moves at all, then it is the sensor itself that has failed.
