I think that you're referring to the block just above the purple part H:
I refer to it as the switch block. Both the rear and the left side of it can get worn. Also, the main switch assembly (the purple part) can get messed up (either inserted or screw in wrong, heated up and warped or the fins can get burned out) can go funky.
Usually, it's not worth throubleshooting...just toss it and get a new one. It's a pain...since you have to rewire. Or if you're sure your wiring is 100%...desolder it from the old switch and solder it onto the new switch.
Best of luck!
Tys