Generically , Body control module (BCM) you are asking about? I am thinking that classic Cascadia has two modules the do that function. The Signal Detect and Actuation Module Chassis "SAM" Chassis located roughly in front of the drivers left foot. Remove the left kick panel and you can see the connectors. That powers the chassis items. There is also the Signal Detect and Actuation Module Cab or "SAM" cab that controls and powers the cab systems and that is located in front of the glove box on the passenger side. Open glove box and remove the four Torx T-25 screws. You can see the SAM chassis from the engine side of firewall and should have a cover, that is where your fuses & relays for trailer lights are. If you post last six of VIN we can get you more accurate information.
I don't have experience with the Columbia except driving back 2006. The manual says There are two power distribution modules. The main power distribution module is the electrical power center of the vehicle and is mounted underneath the electrical cover inside the upper dash assembly (right-hand panel). Also,
there is also an auxiliary power distribution module mounted on the inside of the electrical compartment door located on the B-pillar on the left-hand side behind the seat.
The auxiliary power module has three horizontal rows:
The top row contains several fuses for the sleeper lighting and one sleeper HVAC circuit breaker.
The middle row contains three relays for the sleeper HVAC.
The bottom row contains ignition and battery power for the ABS and data logging unit (DLU).