Because the Rear Integration Module (RIM) communicates with the Body Control Module (BCM) .
Each of the 3 body control modules integrate a number of functional systems under the control of a single module. Each module is connected to the Class 2 serial data circuit, many of the control signals are implemented by Class 2 messages.
"Do any of the devices you described, or the wires to them affect all the six switches together, or is it only the RIM?"
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" What test software would the dealer have that a local ase cert mechanic can't get? "
This will depend on the Shop & if they have acess to Updated Software & Tech 2 Scan Tool. (most don't, because it is very expensive)
(personally) Dealer is still your best way........sorry
Aftermarket & Factory Electronics
Ford, Chev Cert, Heavy Line Mech, Mobile Electronics.Sales, Installation & Repair for 30 years.