[Mondial]

Ron, along with checking for the +23v, you might also check and see if the transistor on the MPX board is pulling the voltage across it to ground.

When there is a stereo signal, the voltage at the collector should be pretty close to zero. If the transistor is not turning on completely there may be significant voltage at the collector which will subtract from whats available to power the LED.

You can also try grounding the wire going to the MPX board directly and seeing how bright the LED lights. This eliminates the switching transistor from the circuit and should result in full brightness if all else is ok.