Fortunately!
Thanks to your persistence, you were able to find the problem without replacing random parts.
A friend of mine had a similar problem with a window lift on his car: it randomly stopped working. After a series of patient searches he was able to figure out that the problem was in a connector that carries electricity to the wires of the window lift motor. If he took the car to an electrician, the first thing he would do would be to replace the window lift motor!