Tachometer question

I working with a 408 that has an aftermarket ignition box and an Autometer tach. I went out tonight and ran some quick wiring to connect the tach using the green wire to ignition box, red to switch and black to ground.
The first thing to tell you is that this tach has been a 2k since I bought it. So tonight I hooked the wiring together connecting the power supply last. As soon as the power wire was supplied the needle went to 10K and stayed there, immediately unhooked didnt see any smoke, no hot wires so I applied it again. This time the needle moved from the 10k to 0. So I started the car and it appears to work correctly and tracked what I would guess was the correct RPM. I only ran it for about 10 seconds. Turned the car off and the tach goes back to the 10k. No I will say I am not hooked to switch on the power wire, just a direct 12v feed. Once I remove the power feed the needle goes back to 2k but not to 0. Again the 2k is where the needle has been since I bought it. Tomorrow night I plan on hooking my timing light/tach up and verifying the correct RPMs.
But before I do that I wanted to check on the needle position. Also should I hook up the tach wire to the - coil side instead of the ignition box. I am hooked to the correct wire on the box according to the box schematic.
Your thoughts?
Thanks Rod