I too fitted an in-line fan just a couple of weeks ago. I removed the tube 
that runs under the belly pan, its just tack welded at the front and clamped 
at the rear, and was surprised to find it in perfect condition under all 
that insulation. I replaced the tube with some large diameter plastic flexy 
tube (from a Mini heater incidently) with a big in-line fan in the middle of 
it. I had to fit the fan off to one side because it is of a larger diameter 
than the tube and the gearshift tube was in the way. It is wrapped in foam 
and just held in place by the belly pan. I wired it up to a spare wiper 
switch I had (the fan is two speed) and it looks like it was always meant to 
be there. It definitely makes a difference to the airflow, but not as much 
as I was expecting, and is barely audible above the general racket of the 
engine. The fan is about 6 inches in diameter and the motor is about the 
size of a coffee mug and it belted out a lot of air when I tested it before 
fitting. But in the van it still only provides a breeze of warm air from the 
vents, even after I checked for blockages. This makes me think that Thom's 
power-supply-fan idea wouldn't be powerful enough. Next, I will try two fans 
in the engine compartment, one into each heat exchanger, and possibly 
drawing air from the interior to see if that works better.

Andy E.

A.C.Erskine@lut.ac.uk