Manuals for ambulances are nearly impossible to track down. If you have anything older, forgeddaboutit (sp??) - it ain't gonna happen. If you have something newer, maybe less than 10 years old, there is a change of getting the builder to make you a new manual. I've been looking for a manual for my 1999 Medic-master since I bought it, and I haven't managed to sniff one out yet, even with a standing offer of $200 as a purchase price offer. Generally, every builder should do things their own way, but there should be some consistency between models and years -- wire numbers should be fairly constant, wire routing should stay about the same, things should be installed in about the same places, etc. Also, if you have an ambulance built before about 2001 +/- (before the digital-everything move took place), you probably have separate relays, switches, and diodes doing the job. Many ambulances had relay panels built by Wired-Rite, since their panels can be set up for positive (eg: signal from another relay) or negative (eg: a door switch) control signals. Their panels have LED pilot lights, so you can flip switches or open doors and see which lights come on.
If you have a digital-everything ambulance, you may / may not have luck tracking down data. If the builder has gone through multiple suppliers, lottsa luck, but I've seen quite a few controls from Akron Brass out there. If you have a digital system, take a look at the Akron Brass web site, and check out the V-MUX section of their electronics products page. Some builders have gone through multiple iterations, and data / parts on their older equipment may be very scarce. Sometimes, you may be able to track down used components from a ambulance company that does re-mounts.
Generally, access to ambulance wiring is through light fixture openings or emergency light (outside) openings. Since the wires and cables are run through split convoluted tubing, you can look to see what color wires are in there, plus they are labeled every few inches with what they are for.
Ambulance manuals are mostly generic as well - they contain material from the vehicle manufacturer - in my case, Freightliner, and it would include the maintenance manual, the driver's manual, and an operation & maintenance manual for the engine (I was able to track down copies on E BAY), and it would have included ambulance material, but a lot of the ambulance material is generic to that manufacturer --- data on the stretcher, data on the oxygen system, data on the seats, data on the 2-way radios, etc. There is actually only a limited amount of data specific to a particular ambulance. Even so, a manual is very nice to have since it is more knowledge about something we don't know much about, but it isn't crucial.
And that being said, I still have a standing offer of $200 for a Freightliner / Medic-Master manual from somewhere between 1998-2001