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.

I would be inspecting the connections at the bulkhead connectors.

Nearly every unusual electrical malfunction on these trucks is the result of poor ground connections. Check and clean them all, add more if necessary.

Hi, I'm going with a ground problem as well. Actually two, the wiper motor isn't grounded well to the firewall and the blower motor isn't either.

Actually also add that the firewall is not grounded to the engine and or battery.

ok, again... why would they wire the wipers different for a diesel equipped truck vs how they are set up for a gas truck?

I don't know for sure but I can tell you the 89 to 91 trucks didn't have a PCM like the gas trucks did. The 91.5 intercooled truck is a read headed step child with a half year only controller. Your son's 92 has a PCM and is different from both the 91's, non intercooled and intercooled. So don't take your 91 service manual for gospel. Either way in the long run why does it matter why they did it that way? It is what it is.

I know that the steering column and wiper switch design had a major change in that time frame but not positive what year the change happened.

Is your steering column round and metal or square and plastic?

The 92 is square and plastic.