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My fathers 2006 automatic convertible is having radio and starting issues. Sometimes when he goes to start the car, it is completely dead and needs a boost to get started. This can happen after a drive or when sitting for a couple of days. Usually at the same time, when this happens his radio does not shut off when the ignition switch is turned off. It’s Almost
as if the radio doesn’t know the car is shut off and opening and closing doors doesn’t work. He has to manually shut the radio off. The battery and starter check out ok. Could this be an intermittent ignition switch problem?
Got to ask..... aftermarket radio? Recently installed?
Agree with @Vette_DD that with a no start immediately after a drive could be an alternator since you state battery's been load tested.
Separate issue imo.
After sitting could be a parasitic draw and if an aftermarket radio, could be the most likely culprit depending on any recent work done.