This is what I have learned so far.....The main Trim Air Valve (controlled by the Trim Air switch in the flight deck) is located in the right pack bay. From there, the hot air splits off and flows to the respective Zone Trim Air Modulating Valves.
The FWD and AFT Cabin Zone Trim Air Modulating Valves are located in the Right Pack bay, and the Flight Deck Zone Trim Air Modulating Valve is located in the Left Pack bay. By the way, all of these valves and ductings are in the front of both pack bays. The trim air ducting for the Flight Deck comes off of the main Trim Air Valve in the Right Pack Bay, passes through the center of the aircraft and into the Left Pack Bay where the Flight Deck Zone Trim Air Modulating Valve is located and attached to the duct. The area in the middle of the bays where this duct passes through is not completely isolated. The Wing Body Overheat sensor loop runs around the whole pack bay on its respective side.