ObRTFM: I've gone over the FAQ and believe I've got everything wired to the same ground potential, but the sonic evidence indicates otherwise. Any additional advice would be welcome.

Vehicle: 2003 Honda element EX.

Factory System: Double-DIN head unit with standard Honda wiring loom, plus three additional connectors: antenna, subwoofer line-out and auto-detect aux-port input (for the in-dash 1/8" connector). Headunit, however, only puts out line-level, as there is a separate OEM amplifier in the passenger kick panel that does 4-channel plus subwoofer amplifying (4x50W max, 1x70W max, not sure of the RMS). The equipment was all manufactured for Honda by Alpine.

The factory system is 100% noise-free. It also plays CDs noise-free even if three of the four "extra" harnesses are detached (antenna, subwoofer, aux-in socket), leaving only the standard Honda/Acura in-dash harness connected (checked that just in case there were ground requirements for the other harnesses).

I replace the OEM headunit with Pioneer DEH-750MP connected through 2 (cheap) two-channel speaker-level to line-level converters and a honda-compatible mating harness. Slight noise ( when headunit's amps are on). Thumps a little bit when amps turn on of off. If I disconnect the eight speaker-side wires, no noise but no music. If I disconnect the "system remote control" (as labelled by Pioneer) from the amp turn on (not labelled), again, no noise but as expected, no music.

Important: Originally, I had the output polarity of the line level converters reversed. They had RCA outs, and I cut off the RCA connectors and wired each shield to the channel's positive and each center conductor to the channel's negative, but the noise was tons (unbearably!) worse then. I took the harness inside, uncrimped, reversed the polarity, recrimped, went back out and hooked it up and, for a moment, I thought I had fixed the issue entirely. But then I noticed the noise was still there, just substantially quieter. I was unsure if that was the 'right' fix or if I should have been swapping the polarity of the inputs to the converters instead, but the wiring up to the converters seemed to be wired with correct polarity. Might have to try swapping the inputs and outputs this time. Or, more likely, replacing these with higher end units? Could this be a channel ground vs. common ground thing?

To be sure, switched back to the OEM head unit, yup, no noise.

Switch back to the Pioneer + converters, yup, noise. I can hear the CD spinning up, alternator whine, etc.

Add the empeg MkIIa (riocar) to the system by plugging its ISO plug into the ISO socket supplied with the unit and wired in parallel with the pioneer harness. This adds substantially more annoying noise (harddrive, processor, etc), regardless of whether the empeg's line level output is connected to the head unit or not.

Add the Parrot CK3000 to the system, by patching it in through the ISO plugs/sockets (described below): symptoms are even worse! Grr.

Pull it all out and put the OEM unit back in. Leave the dash, subwoofer cover, glove box, etc. all in pieces. Sweet music. Fall back, regroup, live to fight another weekend.

My approach on the wiring was to do things as modularly as possible, with no cutting of the OEM or any product's wiring. In order to support the CK3000, I needed to use the speaker-level outs of a new headunit to feed the ck3000, then attenuate the output down to line level before feeding that to the OEM amp. It also required including a conversion of the OEM wiring to ISO plugs, and a conversion of the headunit/empeg's wiring to an ISO double-socket.

While I felt this next item may be important regarding the noise problem adding the empeg, it doesn't explain the above problem with using just the new headunit: the empeg's power/control harness is wired in parallel with the headunit's harness to the double ISO-socket using the extra ISO plug included with the empeg. I've checked the connections of the ground, power and ignition wires connections (all three ways) with a multimeter, while wiggling, all seem to be good and resistance free.

I've done whole system continuity tests from the 'to OEM-wiring' end to the 'to empeg' or 'to pioneer' ends. No problems. How concerned should I be regarding the other wiring vs. chassy grounds or the line-level adaptors above?

Here's a (helpful?) diagram (without Parrot CK3000):

http://brendan.org/element_wiring.txt

Thanks for looking,
Brendan