Are you SURE you want an ammeter? Voltmeters are usually a better solution. That is why automakers changed from ammeters to voltmeters years ago.
With an ammeter you have to have the primary feed for the electrical system running through the guage - dangerous and if the gauge fails your lose all electrics.
Voltmeters are in parallel with the system instead of in series. Much less current and less risk of a failure affecting anything else.
I believe that you have to run a wire from the battery to the ammeter, and then all other feeds to the car (except the big starter wire) have to come from the other side of the gauge.