Routing path and high quality cables are not mutually exclusive. They both can help if it's an inductive pick-up issue. By far the most effective solution is likely to be the cable routing. So you should be able to get away with a cheap RCA cable. For the "experiment" I'd buy a long one that you can run at least a foot away from any current-carrying wires.

If it's easy, you can also try repositioning your current RCA cable while the engine is running to see where it may be picking up noise. A fraction of an inch or a few degrees of rotation can have a big effect.