The replacement fan will require you to carve a bit into the housing. The kit from IPD includes a new fan, switch and resistor pack. You'll need something like a dremal tool to modify the heater fan housing for the new motor.

If your car has the original heater fan it should be replaced, even if it's still working. It won't be much longer.

Here's some pictures of the heater core replacement in my '80 245. This should give you an idea of what's in store:









A collection you'll get in your back seat:



Take the opportunity to clean the HVAC unit! You will be disgusted at how filthy it is. Just think of how much gunk you were blowing into your face over the years!



This is a great way to get all your dash vents cleaned up as well.

This is not a job for the weak of heart. Be ready to sacrafice 4 days of your life just working on your car.

I used it as an oppertunity to swap the dash in my '80 to one of the later styles.

Anyhow, you'll have more questions as the time gets nearer. At this point I can do a heater core from memory, so feel free to email me if you have any questions.
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Formerly 'HearToTemptYou'