In the past, I opted to use the money that would have gone to additional interconnects for a one high-quality pair of interconnects, to help with noise introduction, etc.

I've never gained anything by having the extra fader control that couldn't be adjusted and fixed at the amplifier (gain, etc).

I say don't bother. Before you put everything back together though, have your interconnects run down to the amp in the path they will take permanently then listen to the alternator noise with the engine running at various speeds.

If you get noise, you can either re-route, or if that's impossible (no alternate paths) I'd spend some money on a single set of good interconnects, then run that new cable through ... your noise should go away.

Greg