Car not always starting
 

Hello all-

I have a 2005 scion Tc that starts when it wants to. I just put a brand new battery. Took it to auto zone and tested the alternator and it checked out. The car starts on its own sometimes, but other times it needs a jump to start, again, with a brand new battery. Some suggested the alternator (can I rule this out simply by the Autozone test reading ok?), a ground wire that is messed up, or loose battery cables. The battery cables are tight. Sometimes when your driving, and turn on a turning signal or the radio, the car just turns off. And not sure if this is relevant, but I had a guy install a radio not too long ago, but I'm pretty sure he didn't touch anything under the hood, only in the dash. Seems the problems began about a month after that.. I like to feel like I'm somewhat good with simple repairs so if it's something simple, I will probably get the manual and attempt it myself.. If I do end up taking it into a shop, I want to have at least an idea of what's going on so these thieves around here don't try to rape me, as they've attempted to before.

Thanks in advance for any help..
-jess

who was this "guy" who installed the radio? Maybe it tapped into something it shouldn't have or cut/nicked a wire that is randomly grounding out.