It is under the shifter cover . There is a write up somewhere on here. Take the cover off where your shifter is- There is in the driver side of the shifter a little slide type switch. It gets gunked up over time. A couple screws and it cafefully comes out. I had cleaned mine several years ago with Q-tips and Alcohol - Then put it back. Good as new. It you move shifter back and forth it should make contact again to start truck. I think it is more of a Park/Neutral safety switch.

1990 Black Monty LS Over 300K' 1993 Isuzu Rodeo LS 4x4 46k (yes only 46K!)1989 Honda Transalp 600 Africa twin,