Whenever you have a long wire, it's very prone to pick up noise...somewhat like an antenna...especially in a noisy environment such as a car. Your signal line that is tapped into the iBus, how are you running it to the S1? Are you running just 1 wire and then grounding it at the S1 box? or a pair of wires (signal + ground)? Running just 1 wire is bad and can be exposed to all types of ground loops as well. Also, running just a single wire (without a proper return wire) can lead to picking up noise from nearby parallel wiring (signal coupling). If you're running a pair of wires, it's much better if you twist them together to minimize picking up noises (look at how BMW twists all your speaker wires to/from your amp). Obviously, the best would be to use coaxial or RCA type cables since the signal wire is enclosed by an outer ground wire shield. Hopes that help (somewhat) explaining some of the ways your connection can pick up noises.