Headlight low-high beam switching

[Jadewombat]

My bus is stuck on high beams, I replaced the turn signal and the turn signal flasher box. Ideas? How does it work, the turn signal arm makes contact and ?? switches something?

[Marc]

Inside the hi/lo relay there's a teeter-totter affair that switches state each time the solenoid is pulsed (it has power to it when the lights are on, pulling on the t/s lever closes a contact that sends a ground to the relay). Ordinarily if the lights are stuck on one beam or the other it's either a failure of the relay itself or something's open in the wiring for the ground signal; since you say you've replaced the relay with no effect that makes it seem likely your problem's in the ground wiring.
Your bus is a `66, correct? Here's a diagram:
http://www.vintagebus.com/wiring/bus-65-usa.jpg
The small brown wire ("S") between the t'signal switch and the hi/lo relay is the ground path. With the headlights on (providing power to Term 30 on the relay) the relay should make an audible click each time Term S is grounded.