G'day Adrian!
While in Qld on my hols recently we hired an Astra Convertible. So I could get the faux cobra experience in Vic we took it out at night. The temp got down to 10-12 c so it was just like a typical winters day in Vic except you needed headlights. Now this thing had windows and a steeply raked screen so I let down the windows and sat up straight to feel that cold, ocean breeze.
Turned off the heating but cranked up the seat heaters. Boy, do those things work; had to turn them down as my boys felt like they were in a sauna
So I say stuff the heater, get seat heaters instead. Who needs a demister in a cobra? Just coat the inside windscreen with antifog stuff for those rare times you do get caught in the wet.
Check out CC's own cobraheat advertiser.
Simple install - 4 wires and two switches on the dash (assuming you want your passenger to be comfortable
)
Cheers