Can anyone help me?

So in February of 2015 my car started having issues it at the time involved the radio turning on and off so that was that in April the car wouldn't start when it was out so a hour later it finally started. We got a mechanic a few weeks later to look at it he said that the alternator was dead and the battery was ok we took his advice and had a non Toyota alternator installed all seemed fine except the radio didn't work as well after installation. Then in October/November of 2015 the car stereo started playing up again also now the inside lights would randomly flash external lights would dim when you put the windows up or down. I went to my mechanic said go to a electrical mechanic who said I cant confirm what's wrong unless he keeps it for a week. A month later I went to my normal mechanic and he said oh it seems like your stereo this year went and put a whole new stereo in that night the car didn't start and the mechanic replaced the battery as it was dead all was fine for about 2 weeks and now its messing up again its worse because the stereo turns off and makes a loud boom then comes back on does anyone have any ideas my parents wont let me leave it with the mechanic for a week and its annoying as hell as the mechanic wont say it seems like the alternator is dodgy