I still think most of the problem is condition of the stock heating system,
especially for nonCanadians.  Have the chilly people tried taking the
heater flapper boxes out entirely, so the duct from heater box to body has
no impediments or leaks at all?  I had to do this on one side, due to fit
of the six-cylinder heater box on the left, and I do not have a heating
problem, down to about 14oF that I know of.  Heater output melts plastic
food packaging if it is left close to the floor vent.  Ok, some of that may
be the six rather than four, but some of the flapper boxes I've seen have
been rusty jokes.  Is yours like that?

Back to the subject -- even if you could get 1500watts in the van electrically,
it's not enough.  Put a hair dryer in the van, sitting in the driveway (with
no additional heat loss due to air leaks at speed, and see how long it takes
to warm the van.  Answer is "forever" and the corollary is that the only way
to work on the van when it's below 20oF. is fast and dressed like the michelin
man.

B.