I just made a mod to my rig that was to add a small voltmeter to my main panel so I know what my battery voltage is doing. This particular meter cost about $5 and fits right in the window where you can see the blown fuse LEDs. I'm loving the mod so far. I could simply squeeze the meter into the window and didn't even need to glue or mount it. It's almost a perfect compression fit in that window. Simply tie the red and white wires together and run them to the main hot DC line. Then tie the black to the main DC ground. It's that easy. You may consider it as well:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
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