If you have the adapter then don't cut it off just yet.
I assumed you have the wiring colour code for the Pioneer. Go to your wiring harness in the vehicle and using a voltmeter find out which wire gives you 12 volts with the ignition on and 0 volts when the ignition is off (with respect to ground). This will be your switched power and ensure that this is going to the proper wire through the harness that your Pioneer calls up. Then ensure the grounds line up.