There's a difference between power antenna and remote connections - the remote wire is on whenever the head unit is on, the power antenna wire only comes on when the radio is on. If you use the power antenna connection for an amp, your sub will only work when you're listening to the radio... Not CDs or aux in or whatever. If you don't have a dedicated remote connection on your head unit, just splice into the headunit's switched power. That way the amp will shut off when you switch off the ignition.