Fiat brava 1999 1.2 16v

I have just got this brava from a used car dealer. The stereo seems to have
a life of its own. Sometimes you can turn it on and off ok. Sometimes it
just freezes but is still lit up. Other times i cant get it to switch off.
And my wife took it to work the other day, and when she returned to the
car, it didnt want to start. She got it going after a while. Is this fault
with the radio draining the battery. And has anyone got any ideas about
things i could do to correct it. I know nothing about cars. Thanks