That was short lived though, as FM stopped working a few minute later, but at least the signs pointed to a faulty connection somewhere, but where?
The circuit diagrams helped, as they showed there should be a 8 volt supply to the FM Module when FM was selected. But there was no 8 volts on the input to the module. I traced back the primary source of the power feed to an integrated circuit, outputting 12 volts on pin 5. Yep, that was fine. Next, that fed a an NPN transistor being used to drop the voltage to 8 volts,. As soon as I put my multimeter probe on the one of the pins to that transistor, FM started to work again
Here is a photo of the 3 pins to the NPN transistor. You can see the left pin has a black void, showing a compromised solder joint.
The other solder joints don't look too brilliant either!