Start with the battery first. The simplest. You might have enuff power for the stereo and lights but may be something was drawing power out of the battery and now not enuff juice to start. Try to jump start or go to Walmart and get a brand new battery for $30 and test it out (return it if you want after you done). Don't jump to conclusions and start replacing stuff and the next thing you know its the easiest thing that you overlooked.