I have one switched power plug (turns hot when ignition is on) by the handle bars for my GPS. I have another plug near my battery I use for charging and the heated liner, hot all the time. The voltmeter is an E-bay voltmeter/USB gizmo with an SAE plug on the end.
Can't say with 100% accuracy how my Gerbring heats up, but I did see a change in the voltage that varied with the liner temp setting. Probably some variables at work here, ie, how long it takes for liner to get to temp, how much power to keep it at set temp, etc. It was a one ride deal with the meter just to get an idea of what's going on, when I saw I can power the liner and ride I got rid of the meter. I guess one of those voltage monitors would probably work better for constant reading but I got the info I needed to move on to something else.