The way the circuit works normally is you push the dash button and then switch into reverse. Then you can let go of the button. The button energizes a latching relay(K3 in diagram) until you shift out of Reverse which drops the relay out. Electrically, there are 3 basic components to check. 1) the dash button, 2) the RIO (Reverse Implement Option) switch (S5 in diagram which senses For/Neutral/Rev) and finally, 3) the RIO Latch Relay (K3 in diagram).
If I was a guessing guy, I'd say it's 1 or 2 but anything is possible. You'll need some basic electrical knowledge and simple meter to dig in and test. You can also buy a Technical Manual off Ebay if you're so inclined.
Good luck!
