I should have used 'noise' in lieu of 'engine whine' to not be specific as the same concept applies.
youre picking up induced interference from an energy source, which has been found on many occasion to be caused by, but not limited to, faulty alternators, faulty batteries, faulty ignition components, or faulty wiring.
Rca's used to be the main culprit until noise rejection topology had advanced to negligible returns, so to be fair, i do not believe you have a problem with your signal patch after 2 replacements.
Does the noise exist with the engine off?
Does it exist with the engine on, but the battery disconnected?
It would be a shame to see you replace the source unit only to find you have the same issue from not eliminating all forseeable possibilities.
Idk, it could even be from an outside source like a cell phone or wifi hotspot signal, or a wideband security system. Rare, but entirely possible.
It down to trial and error at this point. The only expense is affording the effort to be certain.