About aftermarket radios, there are several types of them, at several quality levels. This is also true for built-in radios.
When, 9 years ago now, the original Ford radio (very low quality, to be honest) in my car decided to chew cassettes up instead of playing them, I decided to swap it with something having both cd-player and mp3 player. So i bought myself a Sony -whatever- and installed it. 2 days later I was bringing it back at the shop because it buzzed in every single mode it had, and it was complete rubbish. I managed to get to choose another one instead, so I decided to play it safe and went for an Alpine. That has been my stereo set for the following 8 years, following me in the first Golf, and I still have it in its original packing ready to be installed if and when the original stero-set in my current car should fail or start to behave strangely. BTW, the audio quality IS still a little better in the Alpine set that in the current one I have, and the only reason I haven't swapped it yet is because with this one I don't have to bring along the front portion of it every time I park the car.