On the assembly line they build the Ram itself, when it is completed
It is sent to a Body Vendor
The Body Vendor may be owned by Mopar, or it may be be another company that is under contract to Mopar & that body vendor would be close to the assembly plant.
The Body Vendor installs the Exterior Accessories, such as,
Hitch Receiver
Side Steps
Cab Clearance Lights
Tonneau Cover
Spray in Bedliner
At the Warren Michigan Assembly Plant, Where they build the Classic Rams ( Gen 4 ) body style
Body vendor is on the opposite side of town, it is called Ground Effects Limited,
https://www.gfxltd.com/
( who is partially owned by Line-X )
Just down the road, in the town of Sterling Heights Michigan
Where they build the 5th Gen Rams
The Body Vendor is on the Assembly Plants property
( it may be run by Mopar Underground, but i don't know )
It may be “MOPAR CUSTOM SHOP OPERATIONS BODY_VEND”
Mopar Custom Shop video
Follow a Ram 1500 Rebel through its customization journey at a Mopar Custom Shop.
www.youtube.com
Had to research a bit to get that info