Actually I went through the troubleshooting guide many times. The power leads run down the passenger side of the car and the RCA's run down the dirver side of the car. I sanded down to bare metal the ground point in the trunk and fastened it with a star washer.

The dash of the Miata is very compact and I am sure that the RCA's are running very close to the fan motor. I will see if I can re-route them away from it.

I even tried grounding the sled separately, and checked to see if the ground wires in the harness were secure, they were, and the extra grounding made no difference.

The battery is in the trunk of the car, only a few feet from the ground point. Do you think this could be a cause of the problem?

My last hope is to install a noise filter in line with the alternator. I already installed this massive noise filter I got from Crutchfield that goes between the battery and the amp. There is also a filter between the RCA's and the amp as well.

To make matters worse, the installer swapped out the Rio for a Sony unit and we found the noise practically disappeared!!

Alas, doomed to have a perfect system in an imperfect environment!

Help!!!!