I like the stinger ones. But
sonicelectronix also has some that would work probably just as good. On my stinger i had three wires, neg, remote and pos. Its pretty self explanatory. I just hooked the remote turn on to my deck and then negative to my deck then ran the positive to the amplifier positive because thats where you want the volts to be read. With the stinger you have to manually adjust it because it wont be accurate if you just plug it in. I just used a dmm, took the cover off and turned the knob on the volt meter until it matched my dmm. Its really easy.
STINGER SVMB VOLTMETER BLUE 3-DIGIT DIGITAL DISPLAY NEW | eBay