Well after 2 years of living with the intermittent problem, I decided to have a look at the device which connects the two wires form the antenna in the rear window to the coax cable going to the radio (located behind the driver side rear piller). I believe that this a combination pre-amp/balun.

I first checked that the coax connection was not loose, then I unfastened the green case around the pre-amp/balun and found minor traces of flux on the circuit board. I scrapped the off the flux and hit the areas which had traces of flux with my soldering iron. Cleaned off the circuit board with circuit board cleaner and ressembled everything.

Now my stereo indicator works the way it should and the reception no longer fades in and out. I verified the stereo indicator by listening to a radio station around 65 miles from my home. While at home the signal is too week to pull in a stereo signal so the stereo indicator goes off and I receive the station in mono. As I drive towards work (in the direction to the radio station) the signal becomes stronger and I receive the station in stereo.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles