Hitch place didn't know where to look!
If it's a full size Blue Bird then the wires are bundled in the D-shaped chase at the top of the left/driver side wall. They run from front to rear in there, around the corner into the rear wall, and down to the lights. Likely crosses the rear exit door over the top, but I can't say I've opened one up there to be 100% confident. Anyway, intercept the wires in that chase on the driver side and figure out a route from there down through the floor.
Since the Blue Bird uses separate turn and brake lamps you'll need a combiner/converter box, same as is used for SUVs and pickup trucks that have separate turn signals. It's basically just a box of diodes. I can't tell you the wire colors off the top of my head but you can remove the brake and turn lamps and find out what color wire connects to each.