Most small hobby "ammeters" measure milliamps, and do not have a scale high enough to use on a car. If you put one inline in the circuit, it will probably fry the ammeter.
Automotive ammeters usually have a shunt arrangement (which has a known resistance) that uses the formulas listed by 302 Z28. The principle is that if you measure the voltage at two points, and have a known resistance between the two points (that is the value of the shunt), the meter can calculate an amperage.
Another option is one of the inductance ammeters, where the meter probes are shaped like two short arcs, and you just surround the wire where you want to measure amperage. They are more expensive meters, but you don't have to disconnect any wires to use them.