So I have a 2003 Monte Carlo ss limited edition. It has onstar and it has an amp In the trunk. I was wondering what kind of wiring harnesses I need to changed the head unit to aftermarket. And any tips would be appreciated

 

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I know old post but here it is anyways

On 07 I bought the wiring interface at Walmart and it worked perfect. I am presuming your 03 uses the factory radio for the "message monitoring system" so it might be different.

One shop told me they mount the factory radio in trunk so you can reset things like oil monitoring I thought you could buy an interface for that.

 

The wiring harness on these is a little more complex than the ones that I was used to prior to owning the car, due to the fact that the factory radio deals with a lot of the serial data communications. There is a decent amount of information on how to do this on the cheap, but there are just as many options on the market that make it easier, especially if you've got an alarm or anything that uses the cars data. When I had my stereo installed, the shop used an iDatalink ADS-AL with the alarm, and there's a little do-dad behind my stereo that retains a "ding" but the speaker is onboard and doesnt use the car's speakers. My friend was really tight on money so we hacked his stereo harness apart, and wired it directly to the aftermarket radio harness. These cars use a factory amplifier for the speakers, so i spliced the wires going in and coming out of it. If you have any questions, let me know.