Stereo probably overheating
. They do that if they're installed in a tiny little compartment with no air flow.
Lights will be on a common circuit; just one wire that runs around the cabin
with each light connected to it in parallel. Probably easiest to check the positive connection first. Take a look behind the switch panel and identify the switch that controls the saloon
lights. Make sure that positive connection is good (usually coloured red). If that doesn't fix it, find the negative (usually black) associated with the lights and make sure that it makes a good connection with the negative colleciton plate. This is a plate that is usually mounted somewhere near the switch panel. All the negative wires from all of your electrics will be attached to it, and then they in turn will be attached to the negative on the battery
(bigger wire) and ground (also a big wire which will lead either to your engine
or to a grounding plate on the hull). Make any sense?