If you want an electric heater do a search on back seat heater
12V 300 watt heaters $75 and up I found.
If you want an alternator kit, search for VW alternator kit.
Sorry don't know how to post links for you.
I have used the backseat heater in numerous cars in the past. If you don't use it diligently you will run down your battery if you just have a generator as it only puts a out~30 amps where alt put out 55 or more amps.