If you ground the gauge wire, the gauge will go to full hot. Unground that one and ground the other one and try for od engagement on the road.

Tested with an ohmeter, the tranny temp switch should have continuity from terminal to ground, and the sender tested between wiring harness connector and ground with a test light should show a flashing light, and gauge should move.

tranny temp switch is the highest and furthest aft, I think. Coolant temp sensor is further forward and lower.

Not responsible for advice not taken...