Well, you should be able to either use the pink wire behind the stereo for the amp turn on or go to the amp itself and probe for the wire that turns on with the radio.
You'll need to get the pre-amp wires on the sub if you are trying to hook it up that way. Also, not sure if the wires going to the amp are low level anyway. You should use this
http://www.tune-town.com/installatio...t_reducers.asp
or something like it.
BTW, what brand and model of stuff are you trying to use anyways?