OK, here's a wild idea that I'll just throw out there off the top of my head without doing any calculations. Why not take a deep cycle battery and a switch or timer switch. (Intermatic makes a timer switch that runs off a battery and it's good for AC and DC) Fabricate a short 6" or so section of copper pipe with barb connectors on each end to fit the heater hose. Buy some resistance wire like they use for injection line heater and wrap the section of copper with it until you reach the desired load that won't kill your battery. Then, wrap that assembly in insulation and insert it in your heater core lines. Next, wire the power from your aux battery to that heater strip and also to your aux water pump so the wire heats the water and the pump circulates the water. If you end up finding that it kills the battery too fast, just add more turns of resistance wire.

As a bonus, you can hook your aux battery up to a device called a battery isolator and then connect it to your main charging circuit. That way, the aux battery won't suck down your main battery.

It'll probably need refinement but that's the best I can come up with in 5 minutes of thought...

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250