It would appear to be the wiring that's the issue, based on your findings. Disconnect the plug blocks, look for corrosion, it's normally green ish. Also examine the female connectors, look for an extra wide space where the male part goes in. If there's one use a small screwdriver to close the gap slightly. On the male side connector block make sure all the spade end fittings are level/square in the block. It's possible one or more has got bent and instead of mating correctly in the female connector/s, it's going over/under which will give you a bad connection
It's also possible either a female or male connector is damage/broken, again this will give you a bad connection.
If it's as simple as moving the block or wires going into the block, it's going to be something as mentioned above.