In this picture, the front of the car is to the right:

IMG_3206 by krshultz, on Flickr

My sense is that the red capped switch is the pressure switch. It has two wires coming straight out of it, a brown/white and a brown/black. These wires connect to the rest of the car at the two, large, white plastic connectors. Here is what I've observed:

1. When I apply my test light to the brown/white wire coming off of the red-capped switch, the test light glows, and the blue K9/1 relay engages. The aux fans do NOT come on.

2. Removing the test light causes the relay to disengage.

3. Bridging the brown/black and brown/white wires does nothing, although the test light still comes on and the relay clicks.

4. I've tried two different blue K9/1 relays, the behavior is the same for both.

5. The fuse in the K9/1 relay has remained intact throughout this testing.

6. Disconnecting the coolant temp sensor on the thermostat housing causes the aux fans to come on at full speed.

Disconnecting the other switch (two black connectors with blue/red wiring) appears to cause the compressor to engage or disengage.

Am I on the right track here trying to troubleshoot the aux fans? I feel a bit out of my element.