The bridge gets its own set of power feeders, and they are reversed in polarity automatically if the model you are referring to is the built-up indexed model. So, it is best to wire the individual bay tracks in the roundhouse with the same left-right wire orientation as the lead to the TT.
I find that the space between the bay apron, its front edge, and the lip of the turntable is the best place to run feeders up and solder them to either the side of the web or under the foot of the rail...your choice. Just let the bridge do the mechanical and automatic reversing for you when you flip engines 180 degrees.
If the model you are referring to is the kit that you must build and that you can motorize, just solder wires to the two rings that go around the shaft at the bottom of the bridge...the pivot. Then when you are running wires to the bus, or main power wires, insert a double-pole, double throw (DPDT) switch in that line, and just remember to throw it hard over before you allow a turned engine to enter the bay...because you will still want to wire the bay tracks with the same orientation as the lead on the opposite side of the TT. Or, if you have the $, use an auto-reverser module from Tony's Train Exchange. MRC makes one, I believe. I am unsure if they work for DC, but I am in DCC and have the PSX-AR from Tony's. Great gift to worry-free trains.