new hu, constant buzz, engine on

Have weird constant buzzing noise, tried all the usual ideas but nothing seems to work, so I thought I'd appeal this awesome group.... * Kenwood DNX7100 HU 2000 Toyota Land Cruiser, factory amp & speakers. HU connected to Toyota adapter harness that ties all pre-out RCA grounds into Toyota signal ground to amp. Amp has floating signal ground input. HU ties signal & chassis ground together. * Constant buzz, all speakers, anytime engine is on. I would guess around 100Hz. Quiet with factory HU or with Kenwood DPX501. Buzz doesn't change with engine rpm, always around 100Hz. * Doesn't seem to be fuel pump: tried diagnostic of key to 'acc', then to 'on', fuel pump is supposed to turn on for a few seconds, but no buzz whatsoever. * HU wired to factory harness chassis ground. Buzz is worse if HU is directly grounded to metal behind dash. * Tried Neucleus noise filters on both acc & constant power. No difference. Tried ground loop isolators -- makes noise worse, depending on their position. * DNX7100 is drawing under 2A from constant +12V. Any ideas? I'm almost ready to give up....

If you're using the ground in the harness, create your own ground. The factory ground is typically substandard for after market units.

Weird. I tried 10 gauge wire from HU chassis ground to grounding bolt next to amp (used by factory harness gnd wire). Didn't change anything. Also tried 10 gauge wire back to batt negative, no change. I'm thinking or running speaker out wires from HU to factory amp, and feeding these to factory amp via line output converter. Does it makes sense to try this? Or would it likely be even worse than using HU pre-outs? Thanks!

One more detail.... There are actually 2 noises: (1) constant buzz when engine is running; (2) high-pitch whine anytime factory dash lights dimmer is part-way on, even with engine off. This one changes pitch depending on dimmer setting. HU dimmer wire isn't connected. Noise (1) is the bigger problem. I can get rid of (2) by turning dash lights either off, or to full brightness. Thanks!