i checked +12 and GND and its a short
That would blow the fuse...
The reason for the question about reversed polarity is that sometimes devices like this are protected from reverse polarity by placing a diode across the power connections
This protects the device, but sacrifices the diode if you connect it up reversed (even for an instant).
This is designed to blow fuses (which it's doing). Unfortunately (if this is the case) the diode is now shorted and will blow fuses no matter which way you provide power.
If there's a warranty issue, get it seen to by a warranty agent. Otherwise, this is the first thing I'd be checking (after making absolutely certain that there wasn't a short for an other obvious external reason.
Remove the power connection from the unit (if you can). See if there's still a short across the removed power cable, or f there's a short across the power connections in the device (you may have to stick probes into a connector).
Check the continuity between both power connections and the case. Unlikely, but possible that the device is designed for the opposite polarity earth.
If you eliminate all of these, it's time to refer to the warranty agent or to try to investigate further (and void any warranty) yourself.