So I finally finished installing tonight. Haven't had a chance to drive car. Test of the setup works good so far though. Valve comes right on with the switch, and shuts off quickly when the car is shut off.
I actually use an Audi air solenoid instead of the one Tony was suggesting. Why? Because I had a couple spare ones lying around

The are of 99 A4 2.8. You can use these, but you'll also need a Bosch 2 wire plug to plug into it. Regular push on leads won't work.
This mod was more of PITA than I thought. Working in tight crammed spaces sucks like no tomorrow.
For getting power to the switch, their is 12 volts switched power right in the AC control head. The fan switch has 1 lead that is hot with ignition. It is the lead labeled C on the back of the switch. I just used that to grab my power for my solenoid.