I lot of good ideas here. Some complicated and some not. I always like to try the "NOT" first. I always keep an electric meter in all my vehicles ($10-$20 cheapies). Any issues, the first thing I check is at the battery. If the vehicle is running you should have 14 or a little higher volts at the positive cable, if so, the same at the battery post. If different you may have a dirty connection there. If lower than 14 votes coming in to the battery, follow others ideas. Next test would be for the battery. Car off, check voltage at cables, than battery, should be same or again dirty cable. If same, check again in an hour or two and if you have a drop, under 12 or just a little over you may have a bad battery. A good battery should read around 12.75 votes.