Replacing the stereo head unit in a 1989 BMW 325i is surprisingly easy to do and can be completed in less than 30 minutes. An upgraded head unit will improve the vehicle's audio quality and increase functionality, especially if you're installing an MP3 or CD player which did not come standard in 1989. In addition, a new head unit will allow you to add additional speakers, such as subwoofers, to the setup since newer head units have pre-amp outputs for subs.
Disconnect the battery from the vehicle by removing the nuts that secure the cables with a wrench.
Examine the dash of the vehicle. Look for grooves between the dash and the body of the car around the radio. The dash is a separate piece that can be pried off. Insert a flat head screwdriver between the dash and the radio. Gently pull the dash off of the body. Repeat this process every few inches in the groove to loosen the entire dash panel. Once the panel is sufficiently loose, it can be removed by hand.
Remove the screws that secure the radio in place with a flat head screwdriver. These screws are located on either side of the radio. Pull out the stock radio. Disconnect all of the wires from the back of the radio then set it aside.
Purchase a wiring harness for the new head unit. The wiring harness will connect the previous stereo's connector wires to the new head unit. The old wires can be hard-wired to the new head unit. The wire color codes for the 1989 BMW 325 are:
Constant 12V+: Red/White
Switched 12V+: Purple/White
Ground: Brown
Antenna Trigger: White
For the front speakers (5 1/4-inch kick panels)
Left Front (+): White
Left Front (-): Black
Right Front (+): White
Right Front (-) : Black
For the rear speakers (5 1/4-inch side panels)
Left Rear (+): White
Left Rear (-): Black
Right Rear (+): White
Right Rear (-): Black
Connect the BMW's stereo wires to the new head unit. Insert the new head unit into the body of the car and screw it in. Reattach the dashboard. Reconnect the battery. Turn on the car and test the stereo.
