I took another look at the picture you posted. Your RF deck is the smaller board on the left. The PWM signal should be one of the 5 wires coming over from the main board on that 5-pin connector on the bottom. Follow the trace from those 5 conectors back to ground/power/IC's ... One of them should be the signal. If you have access to an oscilliscope, you should be able to pick it out easily.

Chris