Possible Fix!!
Okay, I was thinking of going to my dealer next week to ask about this. I have a V6 with standard radio, VIN 286xx, build date 9/03. The RDS displayed when I picked it up, but stopped after I set my presets. So, I went to the garage to do an experiment:
I removed the radio fuse (#11, passanger side), waited a few seconds, then put it back in. I expected to loose my presets, and also expected I would need to enter the radio security code.
After re-inserting the fuse, I turned on the radio. I did NOT have to enter the security code, and my presets were preserved. But here's the good part - I now have RDS info displayed!
I scanned through a few channels, some displayed RDS while others didn't (as expected). I still wanted to reproduce the problem, so I set a new preset. The RDS info remains!
So, I'm happy for now. I'll pay close attention in case it goes away again, so that if it goes away I can try to correlate what I've done to the loss of the RDS display. If it does go away, I'll try the fuse again.
But this may be the magic I was looking for. If anyone else has the standard radio and no RDS display, I would sugest trying this.
(P.S., RDS is displayed in the radio face only - it does not appear on the monochrome MFI display, though the radio frequency and preset number do. I wonder if this could be VAGCOM'd to display in the MFI?)