i just tried to install my new cd player with a digital interface GMOS-09 wiring harness into my 2001 Bonneville SSEI with onstar, but when i got it all put together and turned the car on there was a noise that was mixing into the rear or front speakers. My brother said that it was frequency noise but it only happens when the car is running and it sounds more like engine noise. anyone have this problem or know how to fix it please help!

I'm no electronics or audio guy, but two things come to mind.

Did you run any part of the harness through the engine compartment? Many sensitive systems can pick up electronic interference from the ignition system or plug wires. As far as I was aware, most wires these days are shielded, but....

Did your CD player come with installation instructions, which often have trouble-shooting pointers for problems with the installation.

Are you running any audio wires next to electrical wires? It may also be the radio, what type of headunit is it?

It could be the head unit itself. The amp I have in my regal gets engine noise on the rear left input no matter what I do. The other 3 channels are fine. Its just one of those things that happens. A lot of decks now come with little boxes in line with the yellow power lead, they filter excess noise. You could try a direct power to the battery and a direct chassis ground. Did you try putting the original head unit back in to see if it did it too? You just might not have noticed it before.

the deck is a pioneer 7800mp. The harness shouldnt be plugged into any engine components, i have the deck plugged into digital interface gmos-09. Yes i have the head unit plugged in and there isnt a problem with the noise. The head unit was just pulled from my brothers car and it was crystal clear when he had it and his 98 regal. Could having a monsoon sound system in the car be the problem?

Ahh.... yeah, Pioneer'* have a humongous issue with sound out of the speakers. Lots of times, it'* the fact that they have a horrible internal ground.

Just recently, I did a custom install (amp racks) on an '03 Trailblazer, had horrible sound issue'*. Ended up being the RCA outputs on the deck were either loose, or not properly shielded, causing all sorts of problems. A new deck solved the problem.

One thing you can try, when you got the radio out of the box, there should have been a white piece of plastic with a screw bolted to the back of the radio. Put a ground lead where that screw went into the heatsink, and ground out the chassis to the vehicle itself. Seems to have fixed a lot of the problems most of the Pioneer'* come into my bay with.