There is usually a block of wood with a hole cut in it the size of the hole in the bottom cab under the metal plate. This spaces the toggle switch into the cabinet a bit so it doesn’t get crushed by pin lifts or hand trucks.

If you don’t care how it looks and just want it functional, you could cut a block of wood to the size of a blank electrical plate. Trace the hole that’s in the cab bottom onto the center of the block of wood and cut it out. Drill a hole in the metal plate for the toggle switch. Mount with 4 screws. All parts available at your local home center.

https://www.homedepot.com/p/RACO-Electrical-Square-Flat-Blank-Cover-8752/100542712#overlay

https://www.homedepot.com/p/Gardner-Bender-8-Amp-Single-Pole-Toggle-Switch-1-Pack-GSW-18/100141285

The wiring is not complicated. Pull those 2 wires apart and one goes to each terminal on the toggle switch. I’m sure you know this but for safety, make sure to unplug the game before touching those wires.

Those others are not 5v test points. They shouldn’t measure 5v.

What are you seeing on TP1?

That’s great. Looks like your voltage problems are behind you. Now the hard part begins. Do you have a logic probe?

How can you tell C33 is bad with a meter?

This is a good guide. It should walk you though the steps.

http://pinball.flippers.info/system6repairpart1.asp

Before you start, might be good to follow this thread. https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6