My experience indicates that the front and rear heaters in the Vanagon are
quite adequate to warm the cabin, even at highway speeds and and
temperatures of
-25C (-13F).

A double cab interior is much smaller than the passenger van, so it should
warm up quite nicely.  However, I am not aware if a double cab has a rear
heater core.  If it doesn't, then the available heat ouput would be severly
reduced.  The rear heater seems to have a greater heat output than does the
front heater.

If the heated air is not warm when the engine is warm, cold air is possibly
leaking in from someplace.  Covering the front air intake (with cardboard
and duct tape) is helpful, but do not cover the small side slits as some
fresh air is necessary or else the humidity from breathing builds up quickly
and the side windows frost over. (BTDT)

There are multiple places for cold air to infiltrate the duct system.  If
you feel any cold drafts while driving, check out their source.  Often the
side dash vents can leak cold air if their duct connections are loose.  They
can be easily fixed with duct tape.

There is also the possibility of a partially clogged heater core.  If this
is the case, the air will be lukewarm only.  The only solution is removal
and repair or replacement of the heaater core.

The other alternative is to do what we do with our 73 Bug; park it until
Spring.

Good luck and keep warm.

Budd Premack
86 Syncro, 73 Super
Minneapolis, MN (Land of Frozen Waters)