Im not sure if you'd want to take it this far, but I have used a DC to AC inverter and ran a small ceramic heater off the inverter. I dont recommend this with a generator, Only with an alternator.
Some inverters can be quite expensive but his is definitely the best way to heat your bug up. you'll need to get a ceramic heater that has a thermostat on it otherwise you'll be rolling windows down and flipping triangles out etc...
Anyway if you go this route you wont need to fix your heater channels
I had used this in my Baja and the heat would run you out if I hadn't wired up an on off switch. However it does work great.
" There is a principle which is a bar against all information, which is proof against all arguments and which cannot fail to keep a man in everlasting ignorance—that principle is condemnation before investigation.” : Edmund Spencer
The Scriptures also reflect this same ideology.
Proverbs 18:13 He that answereth a matter before he heareth it, it is folly and shame unto him.