Car Audio Discovery

dirty651.... the "solution" you came up with should be looked at from a diagnostic point of view only..... by grounding the shield of the RCA cable and the noise going away, you simply confirmed that you likely have an open ground in the audio pre-out section of the head unit.... a proper fix would be to take the HU apart and repair the open ground foils, not leave the wire connected to the RCA's....the pre-out section audio ground paths are usually filtered and buffered with SM isolation caps which you have now bypassed....

kudos on being able to track down the problem yourself!!    but keep in mind the reason you dont see this given as a fix to try is that it isnt really a fix... it's a backyard barney DIY fix.... i think most advice you would get about this is to diagnose where the problem lies (which you have done) and then from there have the actual problem corrected.....