first thing is to work out what power the fuse is , is it 15amp/  10 amp / or / 5amp?  what power fuse  is the manual saying for the back window heater?   each fuse is color coded, as to their amp, so find out what amp your heater is useing , , if its 15 amp , check all 15 amp fuses, one at a time with the engin off, and yes you can handel them, take them out one at a time , and look for a broken wire  that runs across the fuse from one pin to the other, you can see it through the fuse body, if its broken then that fuse is  gone , replace it with the same amp /color  amd then try your heater,,
                 m
                
 September 2011