On mine....
Brake controller, 12v Step-down transformer, relay, and circuit breaker are under the dash on the driver's side. Dedicated lines from the batteries are run to the step down transformer.
Relay for brake signal is activated off the air booster switch. I put a Y connector and fuse there.
For trailer signal lights I simply put Y connectors off the wires going to the truck lights and ran each wire to the civi-trailer connector through its own small step-down transformer. No relays at all except the one for the brake controller. The transformers are waterproof and mounted inside the hitch frame, so if the hitch comes off, the transformers and trailer wiring goes with it.
I did have to convert some female rubber shell connectors to male ones to get the Y-connectors to work.