I never said anything about filtering the power lead at the audio gear - they usually have an LC filter just inside the metal case.This presumes that the problem is RFI; alternator whine isn't. It is at audio frequencies.
I have tried "filtering" power going to audio and two-way radio devices with simple bypasses (nF to 10,000uF), and LC filters. Never resulted in an improvement, except in a couple of cases.
The whine probably isn't getting in by the power lead anyway.