I've been lucky enough to read a how a few other people have installed thier heaters so I've just basically copied what they did, no point in re-inventing the wheel as they say !
This is the heater, the difficulty in mounting this into the wheel arch cavity is that the mounting flanges on each side are very difficult to reach when it's in position. I understand this is even worse in the MKIV body shape which I have here. Standard fitting involves using self tapping screws to hold this in but I really can't see how you manage to get your hand in to tighten them up. So here follows the alternative tried and tested method of fiting the heater.
Firstly remove the flanges, this job was completed using a cut off wheel in the dremel. In truth it's only one side that causes the problem but the alternative solution is so neat I decided to apply it to both sides. Here you can see the flanges on the end caps which have been removed first to make the job easier
Heater temporarily reassembled to show how it looks with the flanges removed
Next, dril four holes into the face of the heater, after you removed the innards to prevent any damage, and fit four Rivnuts.
Make sure that the position of the rivnuts does not interfere with the heater matix. My holes for the lower rivnuts were drilled in line with the holes for the self tapping screws holding the heater outlet plate on.
Make a template of the heater face, carefully marking the position of the rivnuts.
Transfer this to the body by holding the template inplace with the heater outlet which is conveniently removeable from the heater body and then drill four holes to correspond with the position of the Rivnuts. I found this easier from the wheel arch cavity side.
Refit the outlet plate, bolt through from the other side and voila! Heater installed.
View from the footwell.
You can see in this picture that the top left bolt hole is a little close to the outer wall of the footwell and with the washers I was using it wouldn't fit. Not the end of the world I'll just have to use a smaller washer on this bolt. This basically occurs because the passenger footwell has a double skin injected with foam as a safety measure which isn't obvious (unless you think about it!) when looking back from the wheel arch cavity the other side which only has a single skin. See picture above. For reference, my holes were drilled in the heater face 20mm in from the outer edge. I'd suggest it ought to be about 30mm, at least for the top left fixing looking at the face to give you more room to insert the bolts.