I wonder if you might be able to get an MP3/satilight radio out of a Scion and if it would just plug right into your HL? I know you can find them all over
ebay and since Toyota owns Scion, there is a chance that it might be plug and play. Just a thought.
My wife has the 06 Scion xB (toaster) so if I think of it, I will pull out her stereo and see what the connections on the back look like and compair them to my HL stereo.
To answer you question about replacing it with an aftermarket stereo, I think you could do it with no issues. In my opinion, if you have replaced one stereo, you can replace almost all of them. They all seem to be very similar. Just get the harness adapter so you can connect the stereo wires to the harness on the work bench and then just plug it into the factory harness. You are good to go.
Good luck.