My 2 cents worth....

Dont fit an ammeter AT ALL

Because.....
1) The length of cables will only drop the charging voltage/current.
2) The cables need to be alot thicker as they have to carry all the
vehicles power (or charging power if wired as only an alternator monitor)
3) Cables need to be as short as possible.
4) Can cause fires in car if incorrectly fitted or wires are shorted.
5) If ammeter is not high quality, it will drop the voltage too much accros it.

A voltmeter will always work just as well showing what is going on.
Plus it is easier to install.