Oh dear, BigBess, I've never had this problem with a Ducato.
Apparently some vehicle radios have built-in protection against destruction in a current surge or something shorting out. Blown speaker? Base turned up too high on radio? High volume?
Maybe you could find out if the speakers are shorting out for a start. Disconnect the rear speakers from the back of the radio and see
if the problem is still happening. If not, do the same with the front
speakers.
If I'm on the wrong track, maybe someone with hands-on experience of this will have a better idea! Good luck anyway.