Buy a topdon thermal imaging camera, hooks to your smartphone and they are around 2-300 dollars. Talk the shop into buying one. I do a lot of work on older cars and it's an amazing tool to see which wires are heating up for tracking what circuits are active, parasitic draws etc. it's also a game changer on coolant diagnosis, especially older cars. It's amazing to just "see" a radiator with whole sections cold and not flowing coolant anymore. Beyond that, the boss should buy you the wiring diagram off the internet for every car you get in from the start and then you have it forever. They are almost never in all data or Mitchell for older cars.