Is their any way to hook up a subwoofer to my 1997 isuzu trooper?

Question

its the 12 disc premium sound package and i dont want to have to buy a new deck?

Answer 1

It's possible, it will just be a little harder.  If you bought an aftermarket subwoofer it would be very wise to get an amp to power it sufficiently- stock stereos usually don't put out enough watts.  You could buy an adapter to hook up the amp, although I think some amps have speaker level inputs that let you hook them up to a stock deck.

Answer 2

You can buy an adapter that hooks into your  speaker wires and outputs it as aux outputs to run to the left and right of your amp.  Then you hook the blue remote wire to the stereo power out of the fuse box.  Please see below for link of what adapter looks like.  The only bad thing is you can't adjust your bass volume on the amp only from your deck.

The best thing to do would be a new deck you can get for pretty reasonable and an amp.