Hi .....On my boat I have a new Kenwood 12volt car stereo, Kenwood
equalizer, Sony CD player and Amp all connected to two sets of speakers
inside and out. All works well but would like to add headphones while
blocking out the other speakers. The stereo is connected to the
equalizer leaving the speaker connections free, from the equalizer
stereo cables lead to the amp and speaker wire connections. I've tried
to install from Radio Shack a headphone plug and even stereo cables
plugged into the equalizer with a output plug for my headphones.....
nothing yet.... any suggestions?

Rig up a volume control to the output from the head unit. Then just flip
the breaker, or install one, that cuts power to the amp and equalizer. If
all the speakers are driven from the amp then cutting power should kill the
sound.

You probably *don't* want to try driving those headphones straight off the
head unit without something else between the two cutting back the volume
level. Otherwise you'll just destroy the headphones.

Trying to use the RCA out connections from the head unit (the radio) or the
equalizer won't work as they're just at line level. You have to use
something that's driving speaker level to get a signal the headphone can
use. But you only need a TINY fraction of that so be SURE to put a volume
control in there!

Under what circumstances are you expecting to use these headphones?