You MUST have one of these to be cool!
Unless the metal work is already designed for it and they bolt in perfectly, don't waste time on oval speakers. If reasonable and inexpensive fair quality sound, then a good 6" round is better than a 6x9 on several levels. If distressing the car next to you and proving you want to go deaf, then 12" or 15" likowatt powered subwoofers work best.
Honestly, I've been struggling with this in my FJ which has a hidoeous sound system wherein Toyota uses the headliner as a sounding board to make noise and call it 'rear speakers.' That is unless you buy the $1800 audio package where they plug in the 3.5" rear tweeters (available for $179 or so as a kit with OEM grills somehere).
An option is small and cheap and has lots of mounting options... these 3 or 4" cube speakers work pretty well. These can be found for about $25 a pair. Fry's Electronics and the local Radio Shack usually have a handful of choices, too.
Then you just need an amp like this ovrkill 50w/cha model for about $60; and I saw some 5W models for under $10 on
Amazon, though 5w may be too little. Just be sure the amp is 12V powered and have a convenient connection for your audio source (Bluetooth or my favorite, 3.5mm plugs).
Most any local audio store should be able to point you in the wrong direction with 1200W minimum with 20" woofers, etc., but their selection should give you an idea what you'd really like. Ad they usually have cheaper items we can all actually afford. Also the Fry's or RS option.
A other few ideas are neckbands ... but most are NFC or Bluetooth based and probably more personal than what you might be looking for;
favi-entertainment-boomerang-wireless-surround-sound-for-tablets-and-smartphones