I once lived where -30' C was a risk your car might not start, and -40 was routine for a week or two in February, so I learnt how to make it start reliably. (Yes, even in -40.) The only time I heard of a water heater failing was from a lack of fluid. The best water heater was the inline hose water heater, which did more to circulate the heat evenly. Rectifying AC to DC won't solve the problem.
All cars sold had 500W water block heaters, and outlets for employee parking at work were also standard. However, one of four timeshares was used, and they had an 800 amp transformer DC starter on a two-wheel truck for emergencies when the plug was not reliable.
All sorts of tools included a toaster + diode in snow and jumper wires line to + of battery and neutral to V- when I was a young poor engineer and had 2 kids. That would take a dead battery and get me started in about 20 minutes at -30'C if I put a hair dryer on the carburetor. Sometimes it took a little diesel starter spray.
We always carried jumper cables just in case of a weak cell or a burnt-out alternator diode that only ran on 1 or 2 phases out of 3. Americans were so appreciative when they got spontaneous help with a dead car in the parking lot, but in Canada, it's normal practice to stop and help anyone. Even hitchhiking to Uni was a normal practice and sometimes with door-to-door service from strangers.
My best advice is to check for leaks, your fluid level and get an inline water heater and follow the installation instructions. It has a thermistor for protection, unlike block heaters.
e.g.
https://evwest.com/in-line-fluid-he...lFzmfsKkSohzqyGcyk8X4OIDOyYSq89hH9TasKiuE_av0
Tony from Winterpeg.