Sounds like a loop to me. Try picking up an isolator from Radio Shack (<$5 I think) and see if that helps. If it does, you can either leave it in or try fixing the grounding.Originally Posted by skippy911
Just because both items are grounded at the same location does not mean that the location itself is a good one. You might need to move it around to find a better spot. Also make sure your ground leads aren't too long from either device, if they are that can cause the issue too (I know sub 12" is the rule of thumb for AMPs).
Where is your relay getting it's power? If it is off the internal system rather than the battery, grounding it and the p1900 at the same point (or close to each other) could be a bad idea.
What happens when you run straight to your AUX-In and skip the relay? (if no issues there, i'd say separate the grounds)