Regarding the radio, my step-son procured a AM-FM-CD-Cassette unit from an '01 Jeep Grand Cherokee. I installed it in place of the dead unit that came with the trailer. FWIW, given what it cost me to install it, it was a much better upgrade than purchasing a new unit. It required some modification of the cabinetry in my trailer, but others may not. Here's a link to the job:
Any one of the AM/FM/CD automotive aftermarket units will work fine, too.
I have the same soft spot in the same location. It's roughly 9"x9" or so. I can't find any evidence of a leak yet. I did re-do all of the lap sealant in that corner when I determined that it was a soft spot. Because of it's small size, I am just going to sit tight until spring and then re-examine it. My initial thought is rather than open up those outside seams, I might just drill a hole in the inside ceiling paneling, inspect from that side, and repair the soft spot by injecting some "Great Stuff" expanding foam into that area. Insert a white plastic plug in to the hole, and it's a permanent fix. I'll place a board and weights on the roof so that the foam doesn't raise the roof when it expands.
It is not plywood under the rubber membrane. Sunline used a backer material that is kind of a cross between cardboard and masonite. Hard to describe. It is not very thick and it is not very resistant to penetration. If undamaged, it is more than adequate for snow loads and other loads that are widely spread. Concentrated loads like a human foot will cause damage. Walking on the roof requires plywood panels to be laid down that span the roof joists. A tarp or similar should be under the plywood to prevent any damage to the membrane. Lots of us use old pieces of carpet.
My personal feeling is to avoid opening up the roof from the outside if at all possible. Dicor offers self-leveling lap sealant and a roof coating product plus repair kits. Eternabond tape in conjunction with lap sealant and/or roof coating material is an excellent repair and does not require any disassembly of the roof/exterior components. If there is a small amount of water damage that doesn't affect the structural soundness of the trailer, I think it is a much better idea to do any needed repairs from the inside when possible.
OTOH, opening up those seams in that corner may be necessary. It's not a real tough job, but it has to be put back together correctly to avoid any possibility of new leaks.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
2011 to present: '11 Cougar 326MKS
1999 to 2011: '99 Sunline T-2453
Amateur Radio kd2iat monitoring 146.52