Fuse Block

Correct that the brake light switch should be always powered (why they can run your battery down if stuck). Thus the supply is a simple circuit, thru the fusebox. Did you try changing the fuse? They can look OK, but the wire be melted right by the cap. I expect much corrosion in IN, so clean and use di-electric grease. As to value, look at photos of pristine fuse boxes (Google Images, ebay). I put one from an 80's motorhome in my 65. Looked very similar but had a few different labels. Most lamp circuits spec a 5 or 10 A fuse, as I recall. The 20 or 30 A ones are for motor loads (blower, ...).