Your bus should start from the rear button, right? The rear start button goes almost directly to the starter. This makes it easy for mechanics to work in back wo having to go close up everything. You have to hold down the switch in the top right corner of your engine door and hit the start button at the same time.
So, then run a wire from your keyswitch to the button in the back. You have now bypassed all of the start relays.
You can rip out all the door/window wires you want.
And pull those pesky buzzers
Myself and several others have done this and it works.
Let us know how it goes.