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.