I find I get head aches if I am in a car with them as GC350z finds, my brain focuses on the heater buy is looking beyond them at the road so I get a head-ache.
Regarding the noise when switching it on, its just the sound of a normal 30A relay. Would be interesting to know what sort of power the rear window really pulls.
If you had access to the relay would be really easy to measure using a multimeter. Once you know that you could work out if you could add another one in safely enough