amp buzzing

OK, here's my little situation with a 2 channel amp and the alternator buzzing/whine in the speakers. I have already read the stickies and will lay out what I've done. My question is what might be next steps if my last options don't work:

2007 ford expedition xlt
Kenwood ddx514/ccd-2000 backup camera/overhead monitor. Using Metra xsvi-5520-nav unit to provide interface with factory harness. Grounded all 3 pieces against main structural frame running through the dash. Everything soldered and/or butt-connected with heat shrink tubing. This setup worked successfully without a problem (no noise).

Decided to add small jvc amp under front seat for front speakers (60 wpc). Installed Polk db651's in front doors. 8 ga power from battery comes in on driver's side and runs around the center console to passenger seat. Ground is attached to a bolt run through chassis under passenger seat. RCA's and remote wire run down passenger side of center console (separate from any power/ground cable).

I get alternator whine/buzzing through the front speakers. This is only when the truck is running. There is no noise with the engine off. If I turn the gains on the amp all the way down, it's barely noticeable with volume at 0. Move the amp gains up to 25%, it's noticeable to almost be annoying.

Here is what I've tried (without success):
Moved the amp ground to both seat posts, making sure it's bare metal contact.
Ran extra 8 ga from battery negative terminal direct to amp
Changed RCA cables (ran RCA's directly to amp without installing it but just laying on the passenger seat floor)
Installed ground loop isolator (just to test, not permanently)

No success with those things above.

Next steps are:
ground RCA's to radio
Run radio ground to either battery or amp location (either one is easy).

After that, I'm out of ideas. Is there anything else I missed besides take the JVC and burn it in effigy? Thanks.

The noise is reasonably loud. If I move the gains up 25%, I need to move the radio volume up to 1/3 before it's drowned out. If I go higher on the gains, it's worse (radio volume needs to be 1/2 or more).

It's a jvc ks-ax3002......had it lying around so it was free. Not the best amp but it's small and fit the application. Though I'm starting to wonder at this point. Never had so many problems with a single amp install like this. The amp sounds fine, quality is ok except for the whine.

My next step was ground the RCA's. Unfortunately, I don't have another 2 channel to throw in there. Only have an Alpine MRP-M500 mono on-hand.......

If you have an ohm meter you can check to make sure the shield of the RCA jacks on the radio are grounded.  There is a chance that the ground path inside the amp is open also.  I am not familiar with that amp, is it bridgeable?  If so use the meter and check the resistance from the shield of the RCA jack that is mounted to the amp, and one of the non-bridged speaker wires.  RCA cable disconnected from the amp.  Let me know what reading you come up with.

The amp is bridgeable.

I'll check with my meter tonight. To check the radio, I disconnect the RCA's and then check the RCA shield on the back of the radio with the frame of the radio, right?

I'll check the amp also (as noted above).

Everything should be powered off, rca's disconnected and I "should" see very little resistance if these things are grounded right (< 1 ohm), correct?

Ok, checked the amp out. With RCA's disconnected, tested the resistance between the RCA shield on the amp and one of the non-bridged speaker terminals. Around .4 ohms on either one. Seems to be ok.

Test the RCA shield on the radio now? Touch the RCA shield and the radio frame? Let me know. Thanks.

I'll go check that one now.

Radio checks out at .5 ohms between RCA shield on back and H.U. chassis. Did see that I was using original factory ground. Thought I bolted it to frame but it's hooked into the harness.

Next at this point?