I always used a toggle to "arm" the transbrake and a button on the shifter to activate it. T handle shifter with a button on the end of the T. I used a two position switch in the ashtray or console to arm the button. Switch in the middle was "off", switched up and the button activated the line lock, switched down and the button became the transbrake. Allowed me to keep my hand on the shifter at all times. Some guys recommend a relay on the transbrake, others don't. I would think since you are using a relay anyway if you use the ground on the neutral switch it would probably work pretty good as most transbrakes are grounded an you supply the power to activate.