No Voltage From Battery

I have a new Interstate battery in my escort and for some reason, there's no voltage coming from the battery unless it's grounded to the negative terminaland the voltage is like 12.6v. I'm trying to hook up my sub amp, but can't get any power to it. Can anyone help me?

umm....yeah...that's how they work.....the negative side of the battery is supposed to be grounded.....am I missing something?

Like Jet-Lee said that is the way it works on a battery system. You must have the battery grounded to the chassis before any voltage will flow. The amp must also be grounded in order for the amp to work. Hook up the amp to the battery, hook up the hot wire and then hook up the negative. You can hook up the negitive first if you wish. As for 12.6 volts that is all you will get from a 12 volt battery unless the alternator which puts out 13 or 14 volts when the engine is running.
If you are trying for a lot of wattage on that amp you may want to use a capacitor to increase and maintain voltage to the amp. as you may see the lights dim when the amp hits otherwise because the alternator cannot keep up with the type of draw that comes from a large amplifier.