Aaron Coolidge writes:

You wouldn't convert it. You'd just hook it up to ~125 VDC. It will work
great. You may have trouble if it has an electronic thermostat (SCR/TRIAC)
because these require the reversals of the AC voltage to stop conducting
(you can turn on a TRIAC with DC across it, but you have to interrupt
the current flowing to shut it off).

Good point. Do any engine heaters have electronic thermostats though?
I was under the impression that they limited themselves simply with the
increased resistance of the heating elements at higher temperatures.

--kyler