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This is a tricky proposition to get right, but isn't difficult to fit with a few precautions and a little bit of a wiring map..
Ammeters can be useful, they can tell you things the voltmeter might not..like a failing battery consuming charge constantly, an alternator (or more likely a generator) which is not keeping up with the demands of the car's lights and accessories. Good for people using large stereos in their car for instance. A drained battery won't drop in volts drastically until it is pretty much flat.
Good connections are made with a little care, and as Dr Mini says hot joints are a threat. Simply solder the terminals, use the same sized wires as the ones supplied, and you can avoid the drama.
An ammeter is wired in series, the terminals aren't +ve and -ve as you would think, but the polarity you wire them in is still of importance. It works a bit like a flow meter showing the direction of current flow down a particular wire. So in essence, you cut the wire, connect the ammeter to the two bared ends and away you go. The direction of flow may be reversed by connecting the terminals the wrong way showing a discharge when the battery is charging, but this is the extent of the embuggerance. Flip the wires to change the indication direction and that is all.
Wire it between +ve 12 volts and ground, and you will have a dead short! Lots of smoke, red hot glowing wires and excitement! So don't ever consider doing this.
Wiring it correctly is easy, making it truly useful needs a little thought. If you wire it between the alternator and the solenoid, and you will only ever see +ve current flow as the alternator operates.
Wire it between the solenoid and the fuse panel and you will only ever see -ve flow as the car's electrics draw current from the battery. So in both cases only half the ammeter's full ability will ever be used.
The ideal ammeter connection sits in line with the battery cable, it will tell you if the battery is being drained or charged, and it can also tell you if the battery is never seeing full charge and is therefore failing. It will tell you if the alternator is broken or if the car is using more than the alternator can supply. It's not a glamorous gauge in use like a temp, oil pressure or rev gauge, because if the car is working absolutely normal, the battery is charged and is in good shape, then it will simply point to zero all day long after the battery has been recharged to 12 volts some time after start up.
To wire it in for this role, you can't just put it in series on the battery cable. The cable is too big, and the starter motor drains 100 amps on start up. To get the same outcome however, you wire the large brown wire from the alternator (normally going to the battery side of the solenoid) to the back of the ammeter using the +ve terminal if it is marked.
There is another brown wire which runs from the solenoid to the fuses, replace this with a wire which runs from this same terminal on the ammeter and connect this to the fuses.
You run another equal sized cable from the second terminal on the ammeter back to the battery post on the solenoid from which you disconnected the alternator wire a moment ago..
After all this, do a test without the engine running. Turn on the head lights. The ammeter should swing to show a negative current. If it shows positive, then swap the wires on the back of the ammeter over. It will then swing to show a positive for a charging current to the battery, and a negative for a draining current.
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SooperDooperMiniCooperExpertEngineering
All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.
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