Just a guess...
... but when a circuit stays hot when you try to shut it off, its usually a relay that's stuck.
Anyway, looking at the defog circuit, there are two relays involved. N10, which is the "combination relay" (includes turn signals, wiper motor too) and K13/1, which is the rear window defog relay.
The circuit is really not too bad and a little time with a meter will find your problem.
N10 is under the hood in the large fuse box near the master cylinder.
N13/1 is in the trunk on the right hand side.
When you say it wouldn't turn off, I assume you mean the yellow light on the pushbutton control unit wouldn't shut off. Unfortunately, K13/1 runs off of N10 in this circuit, and its not clear which relay generates the signal back to the light... so, that's not much help. You'll have to troubleshoot it unless you have known, good relays to swap and see.
I'm gonna guess its N10 simply cuz its the largest, most complex, and undoubtably, the most expensive. :-)
Hope this helps some, KenP