Phill,
I have a copy of TM 9-741 (Medium Armoured Car T17E1), which is the Tech Manual for the Stag.
I've looked through it, but there are no wiring diagrams as such, just a bunch of smaller task oriented electrical diagrams. You probably know the sort of thing I'm describing.
If you're coming to the big smoke, you can have a look and borrow it, if it's not already in your collection.
Payment would be something of little value, like say a Model T engine
Jack