...blinker probably won't work until I get all the lights hooked up to draw enough
power to trip the switch
This is an age old problem with light duty vehicles like passenger cars. The function of the
blinker relay (flasher) is dependent on the electrical load. Back in auto mechanics school
(a few centuries ago) it was explained that this is deliberate, so that you will notice
when one bulb burns out -- the blinkers will not blink.
Conversely, if you add bulbs, such as when hooking up a trailer, the blinker will blink far
too fast -- often to the point that the bulbs never reach full brightness. The routine fix
for this is to install a Heavy Duty flasher.
A bus should already have a HD flasher -- unless somebody substituted a car flasher
somewhere along the way.
I swear by a SOOOPER HEAVY DUTY flasher used in various commercial vehicles. Signal-Stat part
number 10-53. It costs about 30 bucks, but besides working well, it makes a lovely metallic
ticking sound that cuts thru all reasonable noise without being the least bit obnoxious
-- keeping me from driving all day with the turn signal on!