2015 Mustang GT doesn't start .
It's completely dead No interior lights nothing . It was in a Garage for a month
but the battery was changed recently . In my country there is no service for
Mustangs so you are my only hope .
Completely dead usually means a short to ground. The wiring will need to be probed to see where the fault is. Find a decent garage that has a mechanic that specializes in auto electronics. Maybe some rodents chewed on some wiring?