Most 80 and newer trucks had the TH350C with lockup. They used a vacuum switch on firewall which activated with high vacuum then through the Brake switch under dash to disengage lockup. You could rob the parts from a junker or just run 12V from Brake switch to toggle switch & wire to the trans switch. I recommend using a new Brake switch for an 80 up truck that has more terminals so the lockup will disengage when you try to stop.
th350 lockup switch
i recently installed a th350 with a lockup torque in my 79 4x4, the truck originally didnt have a lockup. so now i gota figure out how im gunna hook up the switch. does it run off of the distributer? or shud i just run a seperate aftermarket switch that i turn on for going down the highway? 
