Applying rear brake kills bike

I just discovered that the reason my rear brake light was sticking on was the rear brake stop switch was gummed up. I tried to very lightly lube up the assembly and everything seemed to be going alright for a couple days. Now the rear brake light barely comes on when I apply brake and subsequently drains all power from the bike. If the bike is off, it'll dim any lights/indicators that are on, and if the bike is running it'll want to kill the bike. It's the original stop switch assembly, so it has some age to it. Could I have killed the switch assembly with wd40 and now when I apply brake it's shorting out the system....or does it sound like something else is going on? The front brake light works like a charm

 

Either/or.

Ditch the WD40, or rather, put it on the shelf and save it for what it's designed for: Water Displacement. Great for drying wet metal; BAD for lubricating things (use it to lube a throttle cable and it could kill you.)

Flush the switch out with contact cleaner or Brake Cleaner and see what happens. You may very well be being "done in" by the WD.

If not, replace the switch. Maybe it has failed internally and you just enhanced its ability to cause bigger problems.