One of the two wires going to the motor could be and probably is shorted to ground somewhere because in the direction that it works that wire is going to ground both at the switch AND at the short point so it is grounded anyway. But when you reverse that connection the lead with the short now has 12V applied to it and the fuse blows.
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2008 Springdale 266RELL
2007 Silverado 2500HD crew cab short bed
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