RCA line noise has been a major issue for car audio installers from the beginning of car audio.
The RCA signal from a head unit is a very low level signal, and it is fairly easy for a noise source like an ECM or other component of the cars electrical system to enter the signal path through the RCA cables. This is even more problematic with today's very low output head units. To make matters worse, different equipment manufacturers use different methods of referencing the signal ground to the main electrical ground of the car. If these methods do not match well they create a "ground loop" causing noise (whine) in the signal.
The car stereo installers can solve the noise problem by tracing all the ground paths to eliminate the ground loop and sometimes they may need to make many changes in the grounds of the different pieces of equipment to find the best solution. Unfortunately this can be a very time consuming process, often taking more time to solve the noise than it took to install the system. So what can be the solution to the problem?