Installing ammeters is problematical and typically ends up in tears for the average DIY(burnt out wires/fires) with limited electrical knowledge. Depending how it is connected up, it may not give you readings that are sensible, especially with all the devices triggered by relays. They were simple enough on older cars where all the electrics came off the fuse panel that mades it way inside the car and the whole power supply was put through the ammeter gauge on the dash.
There are more sophisticated ways of doing it now, but the after market ammeter that you buy would still require it to be done like this.
You can establish how well your system is working from good resolution voltmeters. to 0.1V.
The one advantage of an ammeter is it can give you a very good idea of the exact state of charge of the battery and/or condition. If the battery has been flattened and on jump starting and subsequent driving of the vehicle where the ammeter shows it charging for a few hours then it is a sign that the battery is in good nic. If however, there is no charge happening immediately after the jump start, then it shows a battery that isnt accepting charge. This can be read on a voltmeter with a bit of reasoning, but there are other variables with alternator/regulator output that can cloud the issue.
On a modern car an ammeter would need to be able to show readings of +/- 100A, the instrument is going to be of no use in finding small leakage currents when the car is switched off etc.
