Stay away from the relay. If the dash light is on the relay is closed and a circuit is complete to channel power to the solenoid.

What is not happening is the solenoid is not firing. Either bad solder joint inside the relay, or problem underneath at the solenoid connections.

So, when you crawl under there, take along a wire strippers and a few 1/4 female spade terminals and a 12 volt test lamp.

With engine OFF, gear in 4th, energize OD so dadsh light is on. Crawl underneath with test lamp only. Remove yellow wire and connect to testt almp then ground lamp. Light?? Yes, get other tools. See below. No, check terminal to see if it has good connection to wire. If it has a good connection, check relay solder joints. Don Foster please step in at this point.

Yes light, continued. Check connections where the wires connect to the solenoid paying particular attention to the ground wire. BOTH ends of the ground wire. Also use a short piece of wire to connect the test lamp between the ground terminal of the solenoid and a ground. A good solenoid will allow the lamp to light, dimly at best. NO light, solenoid replacement time.

Educated guess at this time. Solenoid connection terminals. One of them is not making contact with the wires comming down from above or to ground.

Duane