I know that factory radio has speed-sensitive volume control. If you check the wiring diagram for your radio, you should find the correct wire. With the CDR-220, the wire is the one plugged into Socket A, pin 1.
Manuals in PDF form:
http://www.beckerautosound.com/soundadviceonline.html
CDR-220 manual:
http://www.beckerautosound.com/om43-4462.pdf