An ammeter measures flow of current, which basically means that all the electrics you want to measure current draw on have to run through the guage......which then also means your guage has to be sufficiently large enough to handle the current or you'll fry it....

If you just want to measure charging amps then it has to go inline on the alternator cable. But if your current draw from all the electrics is greater than what the alternator can supply, you will get the impression it is charging cos the guage will read 90 amps but if the rest of the car is drawing 110 amps, then the battery has to make up this deficit and thus goes flat if you know what I mean...