[Mondial]

Hmm, that's a bit odd. I didn't notice the connection from the stereo lamp to the function switch, but it may have something to do with the low audio.

Did you use the same circuit with the shunt and series resistor with the LED replacement? It seems something other than Q406 is completing the circuit and causing the LED to light, because the transistor should be definitely off with only millivolts on terminal 4D. It should take at least 1.2 VDC at 4D to turn on the transistors and make the LED come on. You can confirm by removing the wire from 4F and see if the LED still lights. If it does, then some other circuit external to the MPX board is providing a path to ground for the LED current.