With an inline fuse your safe. What really matters is the source being 12 volts. The one thing I don’t know is how much current the 820 is supposed to draw, so you still need to find out what size fuse to put inline. However, I’m not familiar with the 820 power source system so I’m not sure how or where any fuse is integrated into the system from the factory.
A quick call to MarCum customer service will answer everything though. I’d rather you hear directly from them. Give Joe a call at MarCum, I know he’s not busy