I posted these to my PhotoBucket account earlier for SoloSol, and figured it'd be worth posting a tech article since the pictures are half the battle. As a bit of background, I decided to keep heated defrost in the racecar because the temperature differential between morning and afternoon on track can be quite pronounced. I used this function at VIR in both June and 2 weeks ago, and it was very nice.
You have to start by removing the heater plenum from underneath the dash. Bitch all you want, but this the only way to do it. While you're at it, you could also gut the AC plenum of the evaporator core. I can post tech on this if anyone's interested.
On to the pictures...
Plenum from the heater core end:
Removing plastic pieces that directed air to other functions (foot, face, mix):
The trick is to fix the doors so that the air only blows to the defrost function, which happens to be out of the top of the plenum. I put screws through the plenum wall on whatever side would keep a door from moving, like here:
The only problem with affixing the door that controls flow through the core is that it is linked to the valve on the coolant inlet. So if you affix the door completely open like I did, you will always have coolant flowing through the core. If this is a major problem, you could always rig an actuator to the valve that is controlled off the blower switch (see below). For me, the interior is warm enough from exhaust heat and no sound deadening that a little heat from the core (if it even gets to me) doesn't matter.
Finished plenum, ready to be installed:
Installing the defrost ducting:
The ducting piece that feeds the side windows:
It started to break off on both sides, plus I don't need it since the windows are always down when driving, so I took them out:
The current ducting setup:
I don't currently have a picture uploaded to show the blower switch in place in the dash, but here is one of it before installation:
The switch essentially closes a loop in the blower motor ground wire.