Need help - 2nd voltage gauge install

There's been quite a few threads regarding the problem with the dash gauge not receiving adequate power. I put a second, "house" battery in my SAN a few years back and want a 2nd voltage gauge to measure its status. I picked up a cool looking Summit gauge that will look awesome in the side panel of my boat. Here's my problem.

The gauge has 3 leads, a RED, BLACK and PURPLE. Since this gauge isn't switched and I don't want it on all the time I plan on using a relay and wiring it to the keypad power. I think I have that part figured out -- hooking it to the +/- of the power outlet on the side panel seems to energize/de-energize the relay the way I'd like it. The issue tho, is how where do I hook up the RED and BLACK leads? I tried as indicated in the attached diagram hooking to the house battery, no dice. Not only did I get no reading, the leads got really hot really quickly. Not good. Any electrical guru's out there have any advice on what I might be doing wrong?