I have no power to start my motor; wondering if my 7 year old 3 stage Guest power selector switch could have failed. I took it off my boat, looked at the inside, all looks good, I gave it a shot of oil, but still not working....can the INSIDE of the switch corrode, and need simple replacement? How can I tell if the switch is OK, a tester of some type, which I have but no clue how to use.

SOURCE: how to connect a 2 bank (4 batteries) to the 3 GUEST switches.

Sorry for the delay. I don't understand why they have the third "on/off" switch. Normally on a boat like yours, the set would be as follows :
Each pair of batteries would be jumped together to make a single large bank. We can then consider them "bank 1" and "bank 2" usually they will correspond to port and starboard.
Now you will take the positive lead from bank 1 and connect it to B1 on the port switch and then jumper it to B1 on the starboard switch.
You will do the same for bank 2 and the starboard switch.
The "common" on each switch will be your positive output to the motors corresponding them to port and starboard.
Now you can run your port motor with the port switch selected to battery 1 and the starboard motor with the starboard switch selected to battery 2.
Your emergency parallel is already built in. If battery bank 1 dies, you can start both engines with battery bank 2 and vice versa. Putting the switche(s) to the "all" position will allow you to start/run the engine(s) form both banks at once.
You can use the third switch to supply the "house" system. Take its power feed from the "common" terminal on either of the battery switches. In this way you can isolate which battery bank you use to draw power from for all of your accessories so there is no fear of you running a battery bank dead and being stranded. For example if your house feed is connected to the port battery switch and you have the port battery switch selected to bank 1 (port bank), should you run the battery dead listening to the radio you can simply start the starboard engine and then select "all" on the port switch. Now the starboard engine will be charging the port battery bank, bringing the house system back to life and allowing you to start the port engine.
In order for the system to work it is important that all of the battery and engine grounds be connected together in a central point (a common buss bar).
Dont forget that you cannot switch battery switches while the engines are running.
Good Luck!

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