New 'Red Dot' Ammeters now available

[Bob.Murphy]

I don't see why that would be. An Ammeter (in theory) has zero resistance and doesn't affect the current in any way. Grounding the case to the frame, and therefore the ground potential of the battery, shouldn't affect its operation.
There is a light bulb in there of course which is wired to the lighting circuit - not to the current being measured (hopefully). Some, but not all, LED bulbs won't work on reversed polarity but an old style tungsten filament bulb shouldn't care.

Unless I'm missing something, but I can't think what that might be!

Bob.