You can change the default USB connection type in the settings. You can also access it from the notification bar when you plug in the phone to the USB port.
Thanks for the reply, but it doesn't give me what I'm really after. That's my fault: I should have more clearly described what I've tried and what the real question was.
I know how to set the USB connection type. The problem is that neither of the two settings I've tried, Disk Drive and Media Sync, works. I was hoping to find out more about how to match the stereo's requirements for MSC or MTP support, whether there's something tricky about how the SD card is mounted, or what else could be causing the radio to fail to recognize the phone's music storage. I can find my way around unix/linux-style file systems (like Android's), but don't know enough about MTP and how to figure out Gingerbread's and HTC's support for that protocol. Or how to diagnose/debug what's going on when the radio tries to connect.