Don't they make an electric heater for old Volkswagen's. IIRC, there was a guy who I used to work with that had an old bug he used for going hunting.
He put some kinda electric heater it, and said it was great.
I agree that you would want at least a 4 gauge wire from the alt, along with a good high amp breaker.
Here was the type heater I was thinking about.
http://www.ebay.com/itm/New-12-Volt-...item337fc9591d
I used to have to use a very small portable home space heater while I worked a job. The vehicle I had to use didn't have any type of heat so I would bring the small space heater, and plug it into any available connection, and it worked great even on 15 deg F nights.