Pioneer or not, I would start with the ground on the HU in troubleshooting. Especially if just using the stock grounding wire via wire harness. I run my own ground and constants instead of using the harness. In fact I will really only use the harness if stock location speakers are going to be powered by the head unit. If that didn't cure it, I would move to checking how good the amp ground was using a dmm.
This should be done while doing the install anyway, but for troubleshooting others, I would start there.