The trick that people where using to isolate the problem was remove
the Input to the amp and inject audio from cd player or some other source.
If the noise is not still there then its further uptream . One guy injected signals
at the Radio Harness and found no noise. When he got a new radio the noise disappeared. This was with no level shifting devices.
Try a different amp as the others have suggested if the noise is still there with and injected signal.