Now you can cut the voltmeter wires to necessary length and solder them into the spots. You have to solder the red voltmeter wire with orange wire (horn power source) and the black voltmeter wire with the ground wire. Originally there is no ground wire in the left hand control housing, but you can route ground wire for the voltmeter from the black wire of the clutch switch (the clutch switch is close to the left hand control housing and the housing has a hole you can use to route the ground wire from the clutch switch). In my case I had already modified wiring in the left hand control wire harness and in my case the ground is connected to the one wire of the high beam flasher button because I use this button to control the motogadget instrument cluster.
Here is the wires you can use to connect the voltmeter.
Install all internal components back into the housing
Secure the wire harness with zip tie.
Attach the wire guide.
Cut a thick piece of soft foam, put adhesive on it and put the foam on the back side of the voltmeter. This foam will secure the voltmeter from moving back.
Attach the housing back on the handlebar and test it. Done!