I need help with 1978 GMC wiper problem-Thanks for reading!

[cue1go]

Wipers went out so I tested the motor and the switch connectors for power and both were fine. 4 hots and 2 grounds. I bought a new switch from LMC, Off-low-high only switch. installed and the wipers would only work on low speed in the fast position. The switch that was in the truck was off-low-high only switch also. Traced both grounds and 1 turned out to be the instrument lamp light that only comes on with the headlights so there is only one ground in reality and it went to a good ground. My theory is the truck was originally equipped with a delay wiper switch. At some point in the truck's life that switch went bad and the owner went to a salvage yard or had another truck with an off-low-high only switch and used it instead of the delay switch. This would explain why the wipers would only work on slow in the fast position. The first position on the delay switch is delay, 2nd slow, 3rd fast. The off-low-high only switch 1st position is slow, 2nd fast. Explaining why the 1st/slow position does nothing because it's doesn't have delay and the 2nd position operates at low speed. What do y'all think or does any have any experience with the same problem?