93 dakota problem

I'm new to this site and having a problem with the Overdrive in my 93 dakota. I have a 4X4 with the 5.2L 318. The overdrive stopped working last week. I had it brought down to a Tranny shop for an inspection and they told me that the on/off switch in the cab was working and that the solenoid was working. They told me that it is trying to go into Overdrive but just doesn't for some reason. They thought it might be a problem with the wiring harness and wanted $75 an hour to investigate. Has anyone run into this type of problem.

did you unplug the switch? if the switch is bad unplug it and it should drop into OD and make sure the sol. is not lose I have seen the 3 t25 torx become loose and bleed of the pressure

Where is the Sol. located for this?