this is for the audio stuff, that you did not install. i'd start by following any black wires where they may lead to a grounding location, the wires may be chaffed, or the ends where they are bolted or screwed to the frame or body may be lose as well and need new connectors.
installed. also look for the power lines and make sure any connections are not corroded or lose. is so cut the bad splice out, soilder and shrink wrap it. pull the head unit and check for grounds back there too, black wires are almost always grounds.
in the front right side of the engine, the trans bell housing bolts is where your big grounds are,(above the starter) battery cables and stuff, make sure its tight, don't go ape on this bolt tho, its a stud on top of a bolt head, that can break off. maybe remove the bolt and sand all the wire rings to make sure they are grounded solidly.
