I just hooked up an ammeter to my series V last week. It requires a bit of rewiring.
Right now, your wiring should look something like this:
Battery --> Thick, ugly black cable to Starter Solonoid
Starter Solonoid --> Thick brown wire to alternator B+ --> Two thin brown wires to fuse box/ignition key
Your ammeter ought to have two wires, one marked B+ and one marked L+. The ammeter needs to be in-line with the circuit, and you want to put the ammeter as far "back" in the circuit (ie, on the main feed from the battery to everything else) as possible, in order to read the net current flow, positive or negative, for the whole car.
Conceptually, you want to have the ammeter between the starter solonoid and any other wiring. You could accomplish this by disconnecting those three brown wires from the starter solonoid and reconnecting all of them to one side of your ammeter, and then connecting the other side of your ammeter to the solonoid. Now the current has to pass through the ammeter on its way from the battery to everything else (when the flow is negative) and from everything else to the battery (when the flow is positive because the system is charging).
As a practical matter, to minimize effort and new connections on my part, the way i hooked up the ammeter was like this: I disconnected the three brown wires from the solonoid, and connected them to each other (with a butt connector). Now the only wire on the + side of the solonoid is that huge ugly black cable coming into it. There's another big ugly black cable going out to the starter, and a skinny white/red striped wire coming off the middle of the solonoid that goes to (I think) the voltage regulator. I ran a wire from the spade lug terminal on the solonoid where the two brown wires had been to my ammeter. The other end of the ammeter should connect to the junction of those three brown wires we connected together a minute ago. But I didn't have a butt connector big enough to get 4 wires into, and besides, I think the original factory wiring of the ammeter did it this way: Connect the other end of the ammeter to the Alternator B+ spade lug terminal. Mine had an open terminal, part of the same piece that the thick brown wire was running to. Remember, the other end of this thick brown wire is now just connected to the two thin brown wires, so it's just a matter of how you want to bring all these together.
BE SURE TO USE 10 GUAGE WIRE IN HOOKING UP THE AMMETER OR YOU RISK AN ELECTRICAL FIRE.
I can't remember now whether B+ or L+ is the positive or negative side of the ammeter, but you could easily switch these if your meter is reading negative (I'm at my office and the schematics are at home).