12v power supply to put output signal on a 7 way trailor connecter.How to wire the LED voltmeter in series with an illuminated rocker switch,so when the switch is turned on,and the light is lit,the volt meter shows read out of the actual measured voltage.
You don't wire a voltmeter in series with the load. Instead you wire it in parallel with the load because you're trying to measure the voltage across the load.
Figure 1. A voltmeter is wired across the load. An ammeter is wired in series with the load.