I am not sure if this is the problem, but the original switch plates were phenolic that did not get soft when heated. The reproductions are a thermal setting plastic that can get soft and even melt. If there is some resistance and a large amount of current the contacts will get hot and get pushed away from the switch arm. Try to locate an original switch plate and then just solder the new wires to it.
There are two version of the switch plate, early and later. Make sure you have the right one for your car.