Well sorry I wasn't any real help on the wiring issue that your going through. Although something Leong said did strike something. If you have a Mode 1 transmitter then the fire switch might be wired up for Mode 1. So essentially it could be wired up on the right stick if that is how the previous owner used the tank. But like Brett said besides opening up the Tx and tracing the wires, the only other option is to plug in some like just a single servo and test the switch on changing out the servo after every unsucessful try until you find the channel that it is wired to. I know that isn't to much of a help but it's all I could up with.

Bill