b393capt wrote:
As I go to debug this I notice:
1) Moving wires, has no effect on hum. (neither louder or softer)
2) If I power off DVD player only, hum remains.
3) If I unplug DVD player (from 12vdc), hum disappears. (even with RCA
connected)
4) If I unplug the RCA cables, hum dissappears

Sounds like a ground loop on the 12 VDC side to me. A galvanic isolator
in the audio RCA cables might fix it. This leaves the video signal
unisolated, but it might be enough for getting rid of the audio hum.
Note that this device is not the same thing you can put on your shore
power line, to prevent electrolysis on your boat's underwater metal parts.

This picture shows what the device I'm talking about might look like:
http://www.biltema.fi/osteri/data/Kuvat/31_441iso.jpg . There are small
audio transformers inside the box, they break the galvanic connection
between the DVD player and TV, but let audio frequencies pass. It should
not cost much more than 20 dollars/euros. You might find one in an
electronic parts store or in a shop that specializes in audio equipment.

Tapio