On the '04 (right) radio antenna module, the antenna enable signal is on Pin 1 of Connector C2 (red wire). 12vdc to ground is good. Pin 2 (Brown wire) is the composite FM signal; measured to ground, it should be 4-5 vdc, with an FM signal present. The coax carries the right radio antenna FM signal from the antenna module to the radio. Someone with an '06 shop manual can verify the pinouts/wire colors for you to see if they match.
. . . Short pause, as Mr. Rogers takes my beer and keyboard away simultaneously . . . .
"Let's talk about some nice things today. . . I like FM. Can you say that, . . . FM? Good. I knew you could."
Here's how XLR FM works (your milage may vary depending on the year of the XLR you enjoy):
The Diversity Antenna consists of two seperate antennas (elements) bonded to the ground plane on the underside of the rear decklid. The antennas are made of 20 gage, twisted pair stranded wire. A left and right antenna module condition the AM and FM signals. The primary antenna, bonded to the right side of the antenna plane receives both AM & FM signals and routes them to the right side module. The left side module receives only FM signals.
When the radio is energized, it sends an antenna enable signal (power) to the right module. When the AM or FM band is selected on the radio, the right module routes the antenna signals (from the left and right elements) to the radio.
When the FM band is selected, the FM composite signal is sent to the antenna module. Modulated on this signal is the strength of the signal going to the radio. The antenna module uses this signal to switch back and forth to each antenna element, mixing the received inputs to create a stronger signal to send back to the radio. A dual-channel o-scope could be used to measure the phase and amplitude of the FM signals as outputs of each element. You may have a faulty antenna module, which would explain why the signal is fading in and out.
An aftermarket radio receiver may not be communicating/powering with these modules, resulting in less than optimal reception. Connecting to the existing coax input would not necessarily mean the primary module is being powered up. This isn't a passive antenna. It's called an ARS (Active Reception System).
The Tech 2 will display the received signal's strength, as an aid to troubleshooting. Very cool.
Tomorrow night: Nuclear Fusion for Dummies!