Custom Auto Sound

[chevyhed53]

After putting up with an inferior Ken Harrison radio in my '67 Chevelle, I decided to take the "classic radio" plunge again with my '56 stepside. I bought a Custom Autosound unit, with their CD changer, their matched kick panel speakers, and I put a couple of little tweeters in the dash cutouts where the ashtray and heater controls once were. I wired everything through a relay and heat-shrink-wrapped all the connections. With the engine off, and on ACC power the system sounds great. Once the engine is running, though, it cuts in and out. I made sure the grounds were good, and tried again with same results. The Custom Autosound manual (which is not good) suggest that for the problem of "elimination," which is I suppose their term for what I'm experiencing, I should install an "LC filter network in the power lead."
Huh? Try googling "LC filter network" or try putting it into the search field at Crutchfield, Parts Express, Del City or even Amazon, and you get ... nothing. Anybody know what they're talking about? I know there was a thread on this topic a few years ago, but the mystery apparently was not solved. Appreciate any and all suggestions--including returning the whole mess to Custom Autosound!