[Old Dave]

It doesn't matter which cable you put the ammeter in just don't accidentally hit the starter when it's hooked up as you will probably let the smoke out of the meter. You can do the same thing with a 1156 bulb (or any single element 12V bulb) with 2 wires soldered to it. If the lamp glows or ammeter deflects you have current flowing from the battery. Start pulling fuses and unhooking things one at a time until the drain goes away. As mentioned above, the horn relay is a good thing to check, glove compartment light, dome light and also the mechanical voltage regulator on the radiator support (left side) if it hasn't been converted to a newer type alternator with an internal regulator.
Good Luck...