I’m pretty sure this is right. Every 92-93 I’ve looked at had it and the only 91.5 that I’ve gotten my hands on I couldn’t find that plug. I know for a fact 89-91 does not have it because they don’t have a crank sensor. To the OP does your truck have a crank sensor?I thought I remember reading that not all 91.5's had the plug, but I know the 92/93 are supposed to.
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It would be for a different tach. The rare factory tach and isspro first gen plug in tach all use a specific signal generated by the computer that’s why we can’t use a normal tach. What your talking about was probably just a universal tach some places used and all you need is key on power and ground plus a light circuit for the back light, then there is its own sensor you mount then glue two magnets to the crank. I don’t have pictures but I’ve seen one before.The 91.5’s didn’t have a factory tach provision, however they had a 3 terminal snow plow provision ground,key on power, and lights. All routed the same as the later 92’s minus the factory sensor pick up. Dealers used the snow plow provision to tie in a factory tach. Pretty rare but If anyone has any pictures of there’s I’d appreciate it. Looking to see how the added crank sensor was routed