rear glass defroster, draining battery Options

[Hanyo]

Anyone have this problem? My celica's battery gets completely drained from time to time, Initially i thought it was the window roll up modules trickling the power away, or the extra fuel cut and start cut relays i have installed.

But after careful inspection, I notice when ever i leave my rear defroster switch on; my car battery would be completely dead by the next morning. I check the alternator and its giving 14 volts so it prefectly capable of recharging the system.

I'm thinking the auto shut off mechnism in the defroster is dead. Anyone have any idea;s how to test this idea?