[CampNBrew2]

Since its 12vdc, the motor is reversed by the switch reversing 12v + and 12v - to the leads of the motor. So the wire that is grounded (shorted to ground via a pinch, cut, etc) will work on retract since the motor is seeing/using that polarity to work in that direction- ie that wire is used as the ground in that direction. Once the voltage is reversed by the switch, now the grounded wire is fed 12v + and it blows the fuse. Try removing the wires from the switch and metering the wires to ground. The motor should only ground through the switch. If one lead is grounded this is your shorted wire.
Sorry if this is unclear. It didn't come out of my fingers like I see it in my head LOL

Edit: I see Bostongone said this same thing above. I left the window open too long and didnt see it.