Ford Expedition Illuminated Door Sill Plates

Is this an aftermarket part?

Yes it is.
The Expedition that I have don't have still plates.
I am concerned about the electrical part.

This is the item I am planning to purchase:

Ford Expedition Illuminated Door Sill Plates Stainless Steel OEM NEW

http://www.ebay.com/itm/2008-2015-Ford-Expedition-Illuminated-Door-Sill-Plates-Stainless-Steel-OEM-NEW-/321547418980?pt=Motors_Car_Truck_Parts_Accessories&fits=Make%3AFord%7CModel%3AExpedition&hash=item4addb83d64&vxp=mtr

Those are nice. As far as the electrical, you need to check if there is a power supply available in your vehicle. If it was an option, there should be one. Since it is an OEM part, it should plug in and work. To confirm that, I would contact the seller. Since they sell them, I'm sure they can tell you exact.