neatolds

i am trying to fix my 1980 olds cutlass calais. the t top leaks so rain and water would come in.every time i would fix one connection another one would be bad. i have to fix every connection in the car would it be cheaper to replace the whole harness or keep on fixing things a little at a time. i also found the little ground wires under the dash were corroded and one was broken so the first thing i will do is change all the grounds and put a fuse in the main power line to the ac fan someone changed it out and didn't put a fuse in. what size fuse should i use? what size tires are supposed to be on the car there is 14 in tires on there now but it sits pretty low to the ground.