I did it a little differently but it wasn't too hard. And then I decided it was terrible so I undid it.
I wired up all four back lights to be brake lights, but people driving behind me complained because they were obnoxiously bright so I disabled them. It seems like a good idea but didn't really turn out to be.
For the front I purchased replacement reverse lights so they're all white and I used the old switch to turn all four on as flood lights when I'm in a dark place and need exterior light. That I kept, and I like having them up there.
Figuring this out was actually pretty easy but I added regular $5 30A 12v car relays to do so.
In my bus wiring panel on the far left were terminals that were labeled and I could find brake lights, turn signals, etc there. A wire from the appropriate 'input' terminal there to the coil of the relay will get the relay to turn on, or you can trace the wire from a switch down to the relay if you want to turn it on manually. You can grab the flasher switch wire that goes to the yellow flasher unit to use that switch, which made it easy. Ground the relay coil to a ground screw right there in that electrical panel.
The power to the main relay contacts can come from one of the bigger wires leading to a fuse block in that electrical panel (or from your house batteries which I did for the front lights) but should be probably 10 awg wire if you are still with incandescent bulbs.
The output of the relay I connected to the wires that came off of the school bus flasher unit yellow box for the appropriate lights. This was kinda easy to figure out because all of the light wires on my bus were fatter pink wires and I could just hook one up to 12V and see what turned on.
If that doesn't make much sense let me know and I can take some pics and draw a diagram but if you're relatively electrically inclined and understand the basics of relays it's pretty easy.