Best bet would be to get properly sized speakers, first, unless you wanna pay extra for custom mounting.

Ex. Front: DB461p

Ex. Rear: DB691

Now, if you're going the lazy route, you're gonna need more than a $500 budget.

Radio ~$215 w/ parts & install, 4 Speakers ~$250 w/ install. We're already at $465 and we haven't gotten to any sub, amp, wiring, or enclosure.

Front: Polk And you will want these since the Camry in front has 6x9 in the doors.

Rear: Polk

Amp: Pioneer

Very simple to install and will sound great. I would also look at a subwoofer too. You need low end from somewhere, but you don't have to be super loud. You can just get a nice little 8 so you can have so low end. Door speaker/rear deck will not give you lows.