[JRTJH]

The two wires from the battery/converter "TO" the switch have to be good and operational. They provide +12VDC/-12VDC to the switch. It's the switch (not the wiring) that reverses polarity to the awning motor. So, if ONE side of the switch is operational, the wiring from the battery/converter TO the switch has to be operational.

The problem is in the EXTEND side wiring that causes the fuse to "blow" when that side of the switch makes contact with the +/- 12VDC coming into the switch. Pressing the switch to RETRACT doesn't cause the fuse to blow, then pressing the switch to EXTEND can't cause the switch to blow (from a wiring fault between the source (battery) and the switch. THAT WIRING IS INTACT and good.

The problem is in the EXTEND wiring from the switch to the motor.

It can't be the motor since it works in the retract mode and the ONLY difference in retract and extend is which polarity is applied to the motor contacts.
+12VDC --MOTOR-- -12VDC (ground) = retract
-12VDC (ground)--MOTOR-- +12VDC = extend

The problem is in the wiring or wiring connections from the switch, through the trailer sidewall, up the awning folding arms, through the roller tube to the motor contacts. SOMEWHERE in those components/locations the wire is pinched, crimped, insulation cracked/scraped and the wire is making contact with the trailer/awning frame, grounding the + side of the wiring to ground.