First thing to do is clean the ground wires. Especially the one from the neg battery post to the engine block/bellhousing. If that one has become non-conductive then chances are nothing else will work, either. Check the ground from the block to the frame. If that one is bad then no lights. Check the one from the frame to the cab. That one grounds everything inside the cab.
Make sure all your grounds are good before you throw money at it.
