If you have ohm meter, this site may be helpful.

http://www.flashoffroad.com/electric...teryDrain.html

Do a parasitic battery drain search in google. Lots of good info.

 

tell them to look at water getting on electrical connector under the hood for the cbus system, had a friend with a newer mustang and had the same issue, and that was the problem. water was giveing a ground to the system