You picked a fun cicuit to trace down. The wiring between the fuse and the directional flasher daisy-chains via 2 other stops along the way and goes like this:
1) 12V power starts at fuse and runs via a red wire to a connection in the harness that connects to the radio; so examine that connection and also disconnect the radio and see if that stops the shorts
2) Then 12V power runs from that radio connection in the harness via a white wire to terminal W on the bulkhead connectors; the engine compartment side of that W connection goes to the reverse light switch on the trannie on a black wire. So check that out for a short in that wire or in the reverse light switch
3) From terminal W on the bulkhead connector, the 12V runs via a black wire to the signal flasher. Remove the signal flasher to see if the fuse does not blow; it coudl short inside to the case.
4) The signal flasher output goes to the directional switch via a red wire. You could pull this wire out of the column connector to see if the short goes away; if so, the problem is in the directional switch.
