1998 Cont Sub Woofer

How do i turn the sub woofer on?? I have never been able to get it working...any thoughts? Not sure if i am just missing a control feature on the head unit or what?

Thanks!

There's a seperate subwoofer amp on the package shelf that may be defective, disconnected, or bypassed. It takes direction from the HU controls and should power up when the HU is powered up.