2001 Grand Vitari, the radio has a mind of its own

The radio has a mind of its own. The previous owner put a knife disconnect switch in line at the battery because the radio was doing funny things. When we turn it on it sometimes functions properly until you turn the ignition off and the radio doesn’t always turn off. In addition when we get into the car and drive away with the radio off it will at its own will come on and stay playing when we try to turn it off. When the ignition is turned off the radio will keep playing. When this happens we have to open the hood and disconnect the battery. When we need to use the vehicle again we have to open the hood and reconnect the battery. I have called a Suzuki dealer and he assured me that they have never encountered this problem although I had a 2004 Suzuki Verona Lx with the same problem, I took it to the same Suzuki dealer and they returned it to me and said there was nothing wrong with it, and yet they say they have not had a problem such as this. On the way home, after picking up the 2004, (it was at the Suzuki dealer for over a week) the radio came on for no apparent reason, and it would not stay off when we turned it off. That particular car was a real peace; I sold it at auction with less than 11000 miles on it. Has anyone else had these problems, if so is there a fix?