Have a 97 Cummins swap over to a 89 gmc 3500 can't figure out why my light dim in and out ?

 

Do you have a voltage gauge?
The grid heater on the 12 valve motors pull 60+ amps and if it is cycling it will make your headlights/dash/voltage gauge dim. I would pull the fuse for the grid heater and see if it still does it and go from there.