I don't think you need to know how the MK IV is wired internally - you just want to know what connectors go where...

If you do a search on eBay or your favorite file-sharing sites for "BMW WDS" you'll find a DVD which has the schematic diagrams for most recent BMW models. It also lists the pin assignments of each connector in the car, and what color the wires are.

This should be enough to get you to the point where the MK IV and display will power up. Be advised, however, that the navigation system talks to many other components in the car (including some that would surprise you, like the seat memory). You may not be able to get full functionality without something like a PC which will respond to (and generate) signals to the NAV computer.