Every ambulance is delivered with a comprehensive manual that covers everything from the vehicle to the module, but these often wind up in the city garage, and can wind up in the trash can when the vehicle is sold - you are very fortunate if your ambulance still has the original manual.
When I started with my Medic Master, I asked an ambulance electrician about ambulance wiring standards - he told me that there aren't any standards, other than the construction standards in the GSA spec KKK-A-1822E. He told me that every company does things their own way, and every electrical installer also does things with their own spin on them. The most you can hope to find is a summary of wire number assignments, and a very general layout drawing of the wiring. If you have a Horton, there's a chance that the factory may have some general info in their files, particularly on your System 2000 controls. An older control system can turn into a ball of worms if you need components, though.
Ambulances are both very general, and very customized, but if you study them long enough, you'll realize that they are all built to comply with the GSA's KKK standard in order to qualify for federal grants for the purchasers, so the basic designs don't change very much. Even unique layouts become familiar to the builders, and as a result, the individual designs don't need a lot of documentation for the electricians to do their thing. The advantage is that builders would have used the same numbering system for years, so even newer days would probably be backwards compatible.
One advantage we all have is that the wiring is labeled carefully (and continuously). It's not as good as an original set of drawings, but it's a lot better than nothing.
In my own case, I took photos of my relay panel and terminal blocks, then started using a flashlight and digging into the wire bundles to see which relay controlled what equipment. The end result will be more accurate than as-delivered information on an installation that may have been modified over the vehicle's life.
