I had a similar issue that showed up when I would turn up the radio too loud. Over time the connections just got some corrosion and weakend the circuit causing voltage loss. I installed a fuse panel under the dash and broke all the piggy backed wires up into individual circuits and the negative wires all went to the negative buss bar on the fuse panel. Accomplished 2 things. No more voltage loss and makes troubleshooting way easier in the future.