If you want to understand how to scale a shunt for an ammeter this might be helpful:
http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/ammeter-design/ Warning: Some math required(not a lot, but some) Refer to "parallel" circuits if need be. Hint: Parallel circuit, volts is common, current is different.
To be fair, the warning about fire from old wire is a good and honest attempt to keep us safe. If you need a 100 amp charging system. Then you need to protect to that level. Good wire, proper gauge(wire size), proper fuse(and/or circuit breaker), and safe routing along with protective coverings. All of that stuff matters. Would you use cheap, small, funky wire in your house? Of course not. Don't do it in your truck.
