The rear speakers of the Elantra sedan are 6x9s. The common part number is 96360-2D001, of a single cone design and not the best. Nominal power rating is 20W and max power 40W. So you probably don't want to pump more than 15W RMS through them. Given your average aftermarket headunit puts out around 17W rms with a 50W peak, they're close to the limit. Not to mention their lack of sensitivity or frequency response.
The canonical sedan rear deck removal instructions are Sedan Rear Deck Removal DIY.
Note that you can remove the rear deck without removing the rear seats. Just fold the seat down and begin with removing the side bolsters. For the first time its probably worth going through the whole thing. You'll appreciate having the work space.
There is more information and pictures here.
Everyone agrees, though, that its a PITA to do. The best advice is to proceed slowly and carefully.
There is pretty much universal agreement that the results are worthwhile, if you've got an aftermarket head unit.