just google the type of car and look for front or rear that will tell you what size speakers you need the most common are 6.5 round and 6x9.

These will work on a 2010 Ford Mustang GT.

Find out the speaker size of your car. You can always ask the Best Buy car audio guy. I don't drive a jeep but my 2013 honda I had I buy a harness so I could install it.

Installing speakers is pretty straight forward, you have to remove door.l panels to get to the mount point and it is only a matter of 2 wires that need to be connected. If you have any mechanical inclination it's simple. As far as fitment do a little research to find out what stock factory installed size is.

The ideal would be the front seat depending on the vehicle

I would start with the rears and if its an improvement then do the fronts later. The rears are mostly for fill and your front stage up front is more important.

Don't know, google the factory specs.

These speakers are not very strong. You are better off spending more on a higher end brand like alpine or blaupunkt. These speakers distort and don't have enough strength to handle the signal provided. I replaced the with Alpines that were $10 more and got much higher performance instantly.