Here's a bizzare suggestion that worked for me and my 1961 single
cab pick up truck many moons ago...

I bought one of those Coleman propane powered catalitic heaters. It
was about 16" tall and about 10" wide. I then made a lockable mounting
bracket for it and put the bracket/heater on the floor of the cab, 
on just to the right of the heater pipe that runs up to the
defroster (yeah..right) vents, and mounted it all up against the
kick panels so that the unit was attached via the bracket to
the kick panel and floor.

In the winter I would take the heater out of the bracket, light
it and wait to the flames to die down, and the heater to glow. If
you have not used a catalitic heater, there is a "mat" which
the gas is forced into and the heat from this mat causes the gas
to burn without a flame. Its used for camping, etc...

Once the heater was "on" I would put it in its bracket and
drive off. The heater was adjustable, and usually stayed on a 
low setting, and a bottle of propane would last about 10 days.

It kept me warm even on ski trips to New Hampshire...I don't believe
there was much of a fire risk unless the van got hit head on and
the heater came into contact with something combustable...

but then again, that was almost 25 years ago...and I was young,
foolish and imortal...

Ric