Take it to your local Tweeter, HiFi Buys, or Sound Advice. :)
or
If you have a 2001 MPV (or a year close to it) this should work. It's really very easy. One thing to note is the Mazda radio is twice the size of the average stereo, so you'll probably need to get a faceplate or receptacle to cover/fill the empty space below the new stereo you install (unless it's a double din).
1) Remove the 2 screws that hold the "face" of the dashboard in place. Both screws are going straight up through the "face" into the dashboard. You should be able to find the screw holes by feeling around.
2) Once the screws are out, start on the far left of the face. Turn the air duct, so you can grab the facing, and pull it out gently. It may take a little effort, but start pulling gently.
3) Next, work your way around the top & bottom of the facing gently pulling it away from the dashboard. It may be helpful to lower the steering wheel and putting the car in Drive in order to pull the facing away completely.
4) Remove the 4 screws that hold the radio in place: 2 on the top, 1 on the right near the bottom, and 1 on the left near the bottom.
5) Unplug the antenna, the speaker/power plug, and the black wire that attaches to the body of the radio (I believe this is the "ground" wire but am not certain).
6) Buy a wiring harness for your Pioneer that matches the Mazda MPV and wire your radio to it. Then it's as simple as plugging in the antenna, wiring harness, and ground wire and reversing the steps above. Pushing the facing in starting at the right and going to the left has been the easiest way for me.