If your bus has 12V headlights, they might be wired to the schematic in this manual; http://www.busconversion101.com/PDF_Files/MCI%209%20Maint%20Manual%20in%20PDF%20format/MC-9%20-%20Maint%20Manual%20Section%207.pdf
MCI switched from 24v headlights to 12v headlights sometime in the 1980's but I don't know when.
If they are 24V headlights I can take a photo of the page in my MC-5C manual, and they are probably pretty similar. One way to tell is if you have the center-tap relay, called the "12V headlamp cutoff relay" located in the AC junction box on the roof of the front luggage bay. If you have that relay your headlights are probably 12 volt. The 12v wiring is not simple to trace, the basic idea is that the headlights are in series from 24 volts so they drop 12 volts each. The "12v headlamp cutoff relay", what I call the center tap relay, is there to supply a 12 volt source to the headlights so that if one burns out the other one will still work. The 24 volt wiring is a lot more straightforward.