To hook up the aftermarket one, what you'll do is hook the + wire from the amp gauge to the batery side of the starter selonoid, disconnecting ALL other wires from that post except the amp guage wire and the battery + cable. All other wires that normally hook to that post will be connected to the - wire of the amp gauge. That's all there is to it ... very simple stuff.
On my '66, since the engine bay is modified to hell and back, I mounted a junction box under the starter selonoid. The - wire from my amp gauge (aftermarket) hooks to the junction box, and I created a Jumper that connects one side of all junctions together. Then all wires that would normally hook to the + side of the start selonoid, or anything else that needs a 12v source, just connects to the junction box.
It is what it is.