[Oldsguy]

Check all the sockets for corrosion or evidence of water intrusion first, every one of them. If you find some, cleaned it good. If you are blowing fuses, next, remove all the lamps in all the marker, turn signal and backup light fixtures (since you had inspected and cleaned them they should already be empty), and then put a new fuse in and see if it blows. If it does, then you have bad wiring or a bad turn signal flasher unit. If it does not, then add a know good lamp, one at a time and see when the fuse blows, when it does that is your bad fixture (socket) and replace it. If you dont' blow a fuse then you might have the problem fixed. See if all the functions work, if they do, great. If not, then you might have an open in the wiring or a bad fixture (socket) that is not shorted but open, replace it. Step-by-step elimination....
Good luck. To find an open wire you will most likely have to do a thorough inspection and remove some panels to find it. A volt-ohm meter would help if you know how to use one..