just throwing this one out there, there may be an internal problem with the radio, and it is eating up power. wouldnt be the first time somebody sold a broke @ss radio on ebay.
use a meter and watch the current being used by the radio while off, also at the battery with the radio connected and disconnected.
+1,000....
I would think a professional such as your buddy "Conrad the professional" would have known to do this type of test when I originally said do a test on the battery. Maybe not since I see your choice of words indicates that you yourself tested the battery. Perhaps everything needs to be spelled out to you "Buddy". And since it has come to that, take the car back to your installer/friend and tell him to give you good customer service and determine the root cause of your issue even if it leads back to a faulty head unit.
On a more sincere note, thx Loud for hopefully making things more clear. Let's see if the op will listen and stop trolling in the bad alternator/ bad battery thread fishing for an answer.