Zarga, maybe I can help with that; this is what we did for a long trip. I used a JBL Xtreme Speaker/Subwoofer. Now this was waaaaay oversized and waaaaaay overkill, as this has enough sound to fill up a decent-sized room, but it was what I happened to have on hand in a pinch and I didn't have to spend any extra money. Preferably any smaller and more portable BT speaker would do (the JBL Xtreme is kind of a large, heavy potential missile to have unsecured in the passenger compartment in case of an accident, but it was on the floor, which would hopefully help a little).
Anyway I paired the portable speaker via bluetooth to the RSE, then plugged the BT speaker to the 2nd row power jack so that that the power never ran down. My wife somehow got both RSE units showing the same movie (actually in our Expy, the driver's seat RSE unit is the only one with a CD player), and it made a 24 hour trip (split over two days) bearable.
Of course if we tried to listen to anything in the front seat it would have been audio pandemonium, but I suppose it could have been done.