I was afraid of getting that response (but fully expecting). Well that's too bad. Looks like the only thing you can do to improve the sound (appreciably with good bang/buck anyway) is to add a nice aftermarket amp and to replace the speakers (preferably with high sensitivity speakers -- Infinity Kapa and some JBL lines have some high db speakers and decent sound quality to boot -- certainly a marked improvement over factory speakers).
There have been some people that were able to install an aftermarket radio and keep the RES functional, I don't remember the exact details. They had a 3rd gen model (Forget the year, maybe 2008) with no navigation and they removed the faceplate and kept the old radio connected to power the RES and added an aftermarket radio and ran a cable from the aux output in the 3rd row to the rear aux input of their aftermarket radio. This allows for full functionality of the RES. I don't remember the exact details, but it was on Odyclub, I'll try to find a link.