The problem is almost always different for every car. You need to get a multimeter with an amps function, let the vehicle sit for 4+ hours, then remove a battery terminal while keeping the meter touching the cable and the post so there is no interruption of battery power and read what it says. Draw should be less than 50ma. if it is not, have someone start pulling fuses one by one until it goes away. That should help you narrow it down. There are many detailed guides on youtube for this procedure. I would recommend like doc olds said to start with the aftermarket radio as that has the greatest chance of screwing up.