I absolutely agree with the sanding down the ground plane, however I believe with 99% certainty that this is not your problem. Had this been a fresh install and nothing prior was in the car, then sure, it could be one of many possibilities. However you mentioned you had a Sony deck prior and no issues.
You now have a control (your vehicle and equipment inside) and a variable (the source). It is not in any way unbelievable to be the source, hence why I posted the link above.
Here is more directed towards your deck specifically. This is a notorious problem with Pioneer decks.
pioneer pico fuse avic - Google Search
pioneer pico fuse avic - Google Search