installing reverse glow gauges?

The gauges came with an inverter and it has to wires to connect to the headlight power supply. A red for the positive and black for the negative. My question is where is the best spot to hook up the wires directly to the wires from the headlight switch? if so does anyone have a diagram of which wires ar what? Its for a 98 gt Thanks

I connected it to the main brown wire that comes out of the center of the harness I hope that works its not dark enough to tell yet