Howdy,

I have been down this road.
Check your terminals and cables, the rot
can creep up inside an insulated cable
and you can not see it. If you see that blue
green aqua color on the main wires, start
cleaning. If the insulation feels funky,
slit the insulation and check inside.

Check all your grounds. Even if they are tight,
they can still be corroded. Dis-connect them
and take some sand paper to them.

Check the main wire from the fuse box to the battery. I also found this to be corroded under the insulation.