Sorry Airhead, you are wrong and DT is right. The +ve feed to the brake lamp is only live when the lever is pulled/pedal depressed. This would result in the grips only working when the brakes are on.
The easiest way to do this mod is to connect into the TAIL light circuit, the wires are easier to get at than the ignition wires. This means that the grips will work when the sidelights or headlights are on.