Initial experimentation with external shunt ammeter

I think you can use the existing red (BAT) wire to the "+" terminal of your new shunt meter. You must break that red wire in the engine bay and install the shunt in series. That is because the ammeter's purpose is to display current flow in or out of the battery. Note that it doesn't measure transient engine bay loads like the starter or horn, since they come straight off the battery. Similarly for any other "relay powered" loads you add, unless you tap their "red" source on the bulkhead side of your shunt. You won't connect the existing black (ALT) wire to the new meter. Instead, you must run a new wire downstream of the shunt thru the bulkhead to the "-" terminal of the meter. It can be a small 22 awg wire, but must be rugged, either in sheath or teflon insulation. Hopefully you have a spare terminal in your bulkhead to use, otherwise sneak it thru a grommet.

I used another approach. I installed parallel-reversed diodes in series with a new ALT to BAT bypass wire (6 awg) in the engine bay. Those flow either way in "forward bias", which is ~0.5 V drop at 50 A, and not much more at higher currents. That is about the drop across the ammeter at 30A. In effect, the diodes begin shunting current direct from alternator to battery as the ammeter reaches full scale, to keep that current out of the cabin or thru the ammeter. Ditto for high current draws from the battery. Anyone wanting details can search for my post years ago "Modernized Wiring ...". Don't use whimpy diodes. Mine are rated ~200A.