I dont have the service manual for the Road Star here on this computer, but looking at the wiring diagram for a similar bike, it looks like the turn signals and brake light are wired directly into the turn signal switch and the brake light switches. I thought they might all be wired to the ECU, but that does not appear to be the case.
But thats good, because back feeding the brake lights with the turn signals wired together should not hurt anything. I think I was going down the wrong path.
You should be able to find the Road Star Service manual online in a free PDF file, maybe even from the Yamaha motorcycle support webpage itself. The wiring diagrams are on the last pages of the service manuals, it would help you trace thru the wires and connectors (it shows the wire colors) and you can see where you are losing the 12V between the fuse and the tail lights.