I'd suggest for a battery gauge you need a switched live and earth. Admittedly there will be some voltage drop because your battery is at the back and your gauge at the front, but mine still reads quite healthily sat in the dashboard at around 14v.
actually don't...... Can someone tell me why the 2 grounds should be kept separate when they could easily be connected to the same ground point on the body?
For your Batt gauge (voltmeter), just use one of the black switched lives. Yes - because otherwise the current through the gauge lamps will slightly lift the voltage seen by the gauge -ve terminal, so the gauge will under read when the lights are on. If you fuel gauge changes when you turn the headlight dimmer to max or the needle "bounces" in time with the indicators, it's the same issue. PS: You can connect the two separate grounds to the same point on the body, but don't join them up in the back of the gauge cluster.
