On the current 3 series with stock nav, the air conditioner control unit is located in the lower pocket leaving 2 din spaces for the nav unit. On my 2004 330ci, the air condition unit is in the upper slot together with the stock radio. I'm interested in installing an aftermarket double din unit into my car and was wonderiing how hard it is to move the air conditioning unit into the lower pocket?

^

it's not that hard - it's a single part from a car with nav.

The problem is that there is not enough depth behind the climate control unit to fit a true dual din unit - there is just a few inches back there. It would require massive modification to the a/c ducting to get a full dual din in there.