There is a 2005 c320 up for parts at the local junk yard and wanted to check if the audio portion (it looks like it has an audio 20) will work in my 2003 (currently audio 10 w/ cd changer)

 

here are pics of the wiring harness from my radio head unit when I retrofitted command GPS, this should help determine if harness is comparible- which I believe are

 

I'm "pretty sure" that the 2003 C230 is before the "face-lift" W230-version that was done on the 2005. You can tell versions by the center display on the dash, between the speedometer and the tach gauge.
So, iirc, the radio's are not interchangeable.
You can verify by the side of the radio's. One is slanted in more than the other. Also, iirc, there is a thread on the radios that are compatible on the W203 2002+ C230 cars, and the years that have the same fit.

 

make sure to check if the car is fiber optic wires; the 2005 has fiber optic

 

+1 - not only is an '03 pre-facelift and '05 post-facelift so they are physically different, but the pre-facelift vehicles are CAN-bus and post-facelift are fiber optic ring.

Bottom line is no the '05 unit will not work in your '03. Sorry. I did find a stock GPS unit for my '03 years ago and honestly if I were to do it again would probably go with an aftermarket unit. One of the major things to me is that updated maps are no longer available (haven't been for years). The unit itself has the nice factory look/fit, but the graphics are definitely dated.

Best of luck in your quest!