Try popping off the rear panel and checking the actual wire connection to the window. Mine broke/popped off at some point. Same symptoms: I push button no lights no defrost but everything else works.
That is exactly what happened to mine. I never fixed it as I never really have the need to defrost that back window though. I would try to figure out a root cause as to why they broke off though before I solder them back together. I dont like rear defrost anyway. I had a friend who used it and the back window exploded. I think their have been a couple threads here recently about it happening too.