I took this photo when I had the main bulkheads removed for wiring service (see Gutted Interior). At the upper left you can see a hole I cut in the headliner molding to run new mast wiring down through the molded channel that holds the main bulkhead. It is on the inside edge of the molded section that the mast support beam fits into.
Re-Wiring the Mast
Headliner Wiring at Mast Step