If you already have a manual switch for step retraction, you would have to see how it is wired up. There would be different wiring for a switch controlling relays VS a switch wired direct to control the step motor.
Number of wires attached to the existing switch ; there may be as many as 6 ; will tell you how it's wired. If there are 6 , you would need to extend all 6 to the dash switch.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.