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.
https://www.ebay.com/itm/LED-Tail-li....m46890.l49286
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.