I was told that one does not simply add power running boards to an Expedition that didn't come with them, especially a base model with none of the frills like a navigation system. The control electronics aren't there.
Depending on how complex the step motors are, they could be fairly simple or a complex task to rig up as a stand-alone addon. Add plunger switches in the door jambs so that opening the door sends power to something to lower the running boards. Do the motors include switches or something to sense when they're fully open, fully closed, or blocked from opening or closing? Or does it rely on sensing a current draw surge to the motor when motion is blocked by an obstacle or end of travel? If it's just using motor current, how does it tell between end of travel and obstacle?
The door jamb switches can't be the sole power supply to the controller. They couldn't retract when shutting the doors. I suspect a Raspberry Pi Zero (the non-WiFi version) would be extreme overkill for the job, but it's super cheap, if you can find one. An ESP32 might do too. In either case you'd also need relays (mechanical or solid state) to handle the motor current.
Instead of a fancy automatic control, they could have the 'analog' method of direct wiring and a manual DPDT switch with spring return to center off, like old power windows used to be before the automotive industry got the idea that everything had to be electronic/computer controlled.
If your Expedition trim level had the option for power running boards, it may be as simple as finding the connectors underneath, removing dust covers, and plugging them in. That's how they have the trailer wiring hookups. Just need the right harness to plug in, no cutting of the truck wiring required.