I've been running sans alternator for about 2 months.
Its easier for me, since I have a diesel truck
(no ignition system to power, large dual batteries came stock; step 3 in the instructable
Vehicle efficiency upgrades (Go 50-100% farther on a tank of fuel)),
but I suspect if you found a place to install a large deepcycle (RV style) battery you could get away with it too. Put in a battery switch so you can keep the old starting battery from discharging (so you don't accidentally get stranded) and then just run off the deepcycle.
At the end of the day recharge with outlet power.
I also added a 5w solar panel which helps, but not very much.
I don't think you can get much useable energy from waste heat with current technology without spending a whole lot of money, but I did just see a write up on a DIY project on that very topic:
Charge Your Cellphone Using Wasted Heat (and Build a Steampunk Wall-E)