You won't be the first or last owner the rear switch befuddled for a while.
Did you use the bundle of spare wires at drivers side rear to center console area. I'm glad you found issue .
Killed two " birds" with one stone you needed a 200A isolator and a big boy relay the bluesea was probably cheaper. Low power draw from latching relay and lifetime warrantee. Someone asked about Bluesea about warranty "restrictions ect. " they were confused , like don't worry if it ever messes up we just give you a new one. It's on my very short list now. I just put new LED rear tail lights wow are they bright. $192.50 for 8 with IL. tax & shipped. I have seen these listed for up to $50 ea. So they were on my list as the PO had brake lights sick and deformed the bases so the get filthy inside. Just spotted these a couple months ago ,they can get as many as you need if quantity is less than needed.
Put all new side markers last fall Peterson reproduced retired Bargmans on ours.
If you get a chance could you take a pic of weatherstrip on your basement doors. POs put D shaped weatherstrip it seems too short . The bottom strip is much taller.