Installing a USB Port?

almillar

londonTiger - there IS a universal (that's everything outside Apple!) standard - It's USB disk mode. You can plug a USB stick into that port, and the head unit can access the files and folders, and any smartphone I've had over the last few years will do 'USB disk mode' which just makes it look like a USB stick when you plug it in.
Having said all that, lots of new Android handsets have a new thing (can't think of the name) and exclude this mode.
You're right that it's a bit of a minefield and you do have to do your research, but to say that Aux is best is wrong IMO - it's the most compatible, sure, but it's the lowest common denominator - if you can do USB, do it!
I also believe 'works with' simply means 'charges' for your iPhone example above, the phone will still work with the stereo, but not charge, due to Apple swapping pins around.

Strider590

I'd go with a new headunit with USB built in..... They can had for a little more than the unlock code on your existing headunit. You probably won't have been given this code, but on a 2009 car you'll need it in the next 1-2 years when you will undoubtedly need a new car battery.