Above or below then???

If you can seal the fan inlet to the bottom of the intercooler so there is no in-leakage from the sides underneath would be better. But mounting it underneath and getting a good seal may be difficult. The simple reason being is that by putting a fan in the intake decreases the intake area and therefore intake air volume. With no intake fan in this case you will get little or no airflow, with the fan mounted on top of the IC you will get more airflow, and with the fan mounted underneath and properly sealed to the intercooler you will get even more airflow. This is why most cooling fans on modern cards have a full plastic shroud between the fan and radiator and are on the outlet side of the radiator.

Cheers

Snowy

At a reasonable speed you will have ram air cooling. At slow speed heat from the engine will rise up around the IC, fan underneath will draw fresh air down through the IC and blow engine heat away from the IC.

IMHO either will block the total air flow though..

on top: if you get air coming through the scoop, it hits the fan and not all goes through

under; if it gets thru the intercooler, then its still hindered by the fan body/engine in the way compared to clear top and bottom..

Either the air would be fairly turbulant, and not good at highway or daily road speeds.

YTMV

I'm pretty sure it won't fit underneath. If you have a look at the underside of your I/cooler (TD6 4.2) you'll see the plumbing from the I/cooler to the inlet manifold basically goes around in an "S" bend and ends up underneath the core area so I very much doubt if it will fit.
Regards Andrew.