In a 1990 BMW 5 Series, the factory radio amplifier is typically located in the trunk, near the left or right side behind the side panels. You may need to remove the trunk liner or access panels to reach it. In some models, it can also be found under the rear seat or near the spare tire well, depending on the specific configuration. Always consult the vehicle's service manual for detailed instructions on locating and accessing the amplifier.
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If you mean how do you connect it to the wiring, then you will need to find an adapter for the factory wiring in the car.
you can install a factory radio if you can find one or you can order a direct replacement from a company called biketronics you can find them on line at biktronics.com it is a better radio than the factory one
If it is a factory radio, call your local dealer.
timing diagram for 1990 1.8 Mitsubishi eclipse
Go to a dealer
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If it is the factory radio, then the pin-outs diagram should be stamped into the metal on the radio it-self.
When fixing or changing a radio in a car, a wiring diagram will help make sure things are connected correctly. A wiring diagram for the radio of a 1990 Dodge Dakota can be found in its maintenance manual.
Chilton manual has directions on removal
maybe try gm or Chevy house to help with that
on the back of the factory radio.