I had done the same thing with a volvo volt gauge and there are actual points that you can hook the meter onto at the back of the spedo gauge pack. The wiring should screw into the same points where you took it out of the wrecked 7 series car - assuming that your gauge pack is the same?

If not then check Ebay every now and then. There is a guy who sells these volt gauges as upgrades and provides a great picture of how the thing hooks up in his posting. There is none for sale now but check it later if you are willing to wait to wire it like the factory does. Problem with wiring the + feed of the gauge somewhere else would be the voltage drop that occurs if you are wiring the B+ far away from the standard factory location (ie behind the gauge pack) and using thin wires.

Tim