Amp

try a low pass cross over switch you can get a scoch one at wall mart worked for me

You can get an in-line low-pass crossover that will filter out the vocals.

If you are using an after market deck then you need to adjust the low pass filter (LPF) for the sub. Make sure it is at 80 or below, on many decks you have both a "rear" and "sub" outputs. Check the back of your deck and see if you plugged it into the rear out, if you did this it would send trough all frequencies where as the sub would send out pure bass. Remember even with the amps LPF on some vocals are still below 80hz so its best to do all the sub controls through your deck. If you have an OEM head unit this will happen unless you get an adapter to filter out the vocals and other audio, a popular one is the MTX Re-Q and should work with all factory head units.

check your frequency knob and turn it to the lowest possible setting, then turn it up until you here faint vocals in the sub and that is the setting for your sub.

check your amp settings
maybe one is turned off

it making noise when swich is on

I think it's a bad switch