Seems like first you gotta figure out the screen thing, when you say you plugged it directly to the accessory line, did you chop of the cig lighter plug and
still used the "egg" or did you just bypass the "egg"?
As far as grounding goes, I cannot help here. do a serach, I kinda of know but I do not want to fry your stuff.
Radio shack sells a "ground loop isolator" for the audio ports to eliminate the buzz. Try this after properly grounding, if that fails.