Rocky -
The amplifier is actually up in the antenna on the roof. The outlet plate with the little red light is simply a power supply, which sends 12-volt power up the antenna cable to the amplifier.
The problem could be in three places - the power supply behind the outlet plate, the amplifier in the antenna, or the cable between them. The power supply is pretty simple and pretty rugged, so I wouldn't expect the problem to be there. However, you might pull the plate off the wall, and make sure the screw-on connections behind the plate are tight.
The cable from the outlet plate to the antenna is actually a pretty good candidate for failure, since it gets flexed when you crank the antenna up and down, as well as buffeted by the wind and rain as you travel. You might check the upper end to make sure it is screwed on tightly. If that doesn't help, you might unscrew the upper end, and use your voltmeter to check for the presence of 12 volts when the red light is on.
If these don't work, then the problem is probably in the amplifier, which is non-fixable. The AntennaTek web site says that replacement parts are easily available.
You'll get a little help (though not much) from
http://antennatek.com/pdf/Repair_instructions.pdf
Good luck
Bill