Unexplained battery drain

If I let my car sit for more than 36 hours the battery will be drained enough that not even the automatic door locks will work. Neither with the key fob nor the switch on the driver's door. I have had the battery and the alternator tested and they both passed the test fine. My friend had suggested to check the radio as what I have is not factory just not sure how to look up the information to determine how much is being drawn when the key is off. But even then I don't think a radio would draw enough to drain the battery that far and that fast. About a year ago, I made the mistake of leaving my phone charger plugged into the aux power port (cigarette lighter for us old school types) and it took the car almost 5 days before enough juice was drained to keep the car from starting. The radio was installed over 2 years ago. Is there something I can test to find out where the drain is coming from? If so, how would I test it? I am fairly handy and can do most of my own automotive work, I am just at a loss when it comes to electrical issues.

Thank you for any help and assistance you can provide.