Have got the same problem,
My radio was changed and the amp was completely removed and everything rewired in
Another thing is the lights on reminder has stopped sounding
Got an auto electrician to look at it but he couldn't get to the bottom of problem
disconnecting clock stopped air con working
Clock seems to be drawing 0.2A.
There was another 0.1 going but removing Radio2 fuse stopped that without anything else going
So at the moment just doing trickle charge every week or so