It looks to me, reading your post, you've done due diligence to eliminating the most common electrical reasons for the shift lock failure, leaving only a few possible external electrical possibilities, such as: an issue with the relay contacts, or wiring to and from the solenoid and relay. My approach would be to get the cover off and using a sharp probe, be sure the solenoid is indeed getting battery voltage across its winding; that its ground and source from the shift lock relay is there.

Reason I stress this, is the solenoid is a simple and reliable component with a surprisingly hefty price tag, and no fun at all to get to and replace.

Mechanical reasons for this failure mostly involve rust or spilled drinks. Here's a map of the electrical connections. The map is from a '91, when these safety retrofit gimmicks were ordained, but the map is the same through '93.



Now the shifter for '93 is mechanically new that year, and a one-year build. I've never had a '93 torn down so here's the parts diagram for it. #47 is your target.



I've never seen this repair/replace described in any manual, and I've got a collection of them. I'm not sure how relevant the following is, but it would go toward showing just how many tools it takes to get to all the pieces from a '91 automatic transmission shifter. I'm not sure they made it that much easier in the '93 and final version.





--
Art Benstein near Baltimore

"If you can't fix it with a hammer, you have an electrical problem." -vwbusman66