So let's follow on with the problems here.
When you remove the step, the van is going to complain. You get a warning buzzer when the sliding door is opened, presumably to let you know that the step is extending. Then you get another buzzer and a warning on the dash every time you start moving to let you know the step is not properly stowed.
As noted above, I managed to eliminate the warning while driving by sticking a jumper in the dangling plug to simulate the "step stowed" switch.
With a little thinking and a little research on the internet, I think I have a more elegant solution. It requires 3 relays. I'm attaching a schematic of how I think this would work.
We start with all three relays in their "at rest" position. I've drawn a line between pin 30 and 87a to show that resting state. This is how the van spends most of it's time, with the step stowed and the switch detecting the step stowed in the closed position.
The two switches I really want to control are in the upper right of the schematic.
It starts by using the power that would normally extend the step to instead drive a relay (bottom left). When the van tries to extend the step, the power intended for the step motor energizes the relay, which also connects pin 86 of the relay directly to the battery (through the relay at the bottom right). That also energizes the relay at the top right, opening the "step stowed" switch and closing the "step extended" switch. That should then cut the power meant to extend the step. But the relay at bottom left continues to be energized through itself and the relay at bottom right.
When the door is closed, the van would apply power to retract the step. That power energizes the relay at bottom right and interrupts the power to the other two relays, de-energizing them both. That closes the "step stowed" switch and the van will stop sending power to retract the step. And we're back at rest in the initial condition.
Feel free to pick apart my schematic, as that will keep me from zapping myself and/or damaging my car.