The two grey wires on the jack are connected to the small toggle on the front panel. When switched, the amp goes from the OD to the Normal channel. Only problem is, the switch has to be switched back off or the amp will not switch channels back and forth when the footswitch is plugged in.

Switch is on the left. Something like this should work for the mod: SWITCH

Replace the jack above with this: JACK It has a shorting bar that will disconnect when the jack is plugged in. So, the blue wire will be soldered to the TIP terminal. One of the switch wires will be soldered to the SHUNT terminal (the small spring bar that contacts the inside of the TIP terminal). Then, solder the black (ground) wire and other switch wire to the RING terminal on the jack. So, when the footswitch is NOT connected, the panel-mount switch controls the channels. When the footswitch is plugged in, it will disable the panel switch and leave the footswitch as the dominant controller. Simple, easy, convenient. Get to work!
Thanks to Grimespace for pics of his amp.