my son's truck, here.

all of a sudden a couple weeks ago, dry sunny day and wipers came on by themselves. He did not touch the switch. have been fighting this issue since. He has a 91 factory manual (from what little I have looked at it seems like wire colors etc match up to his truck)

I don't see any mention of this in the "wipers" section of said manual or the heat--A/C section but the wiring diagram clearly shows the heat--A/C and wiper system having common wiring, called out on wiring diagram as "DIESEL ONLY".... What would having a gas or diesel powerplant have to do with how they wired these circuits in a given truck? What is different about a gas engine truck where they didn't wire them like this?
Anyone out there ever run into such an issue?

He went thru the flow chart and jumpered everything as it said to and came back to "replace wiper motor" which he has done 3x, 1st with a used one that came from a gas engine truck, and twice with a new reman, thinking the 1st one was "bad"//// still does it. also replaced "multi function switch" with "STANDARD" brand unit from parts store. No change.

now today he showed me that as you push the different buttons on the heat/AC switch, the wipers would come on depending on which button he pushed on that control, again w/o touching the wiper switch.