Its been a while since I've tried so not sure it will works or not but when you plug in your device you have to switch the USB setting to removable storage mode this way the radio just thinks its a USB stick. It should only work for music on ur phone not Pandora or anything streaming.
That's just it, since ICS, Android devices don't have an option to use "removable storage" mode via USB anymore. I believe this was to prevent having to mount and unmount the SDcard storage to use over the USB connection, and eliminate the need to separate app and storage partitions.
You could still try to use it in this manner, just make sure the "MTP" option is enabled (not sure where this would be on an HTC device), and it *may* be able to read it.
EDIT: And by "use it", I mean strictly as a USB device for playing mp3 files stored on your phone; as doc31 mentioned, you wouldn't be able to stream Pandora or anything like that.
I think in order to make the radio think it is a USB stick the phone has to have an external memory card...many years ago, I briefly had a Samsung Captivate and it worked as long as the music was on my external memory card...if it was on the internal one, no luck. Since the One only has an internal card, I know of no way to let the radio access the files. Hope I am wrong though because that is one feature I miss too.
David
In MTP mode, you'll get a directory for both the internal storage and the external SD card (if present), so that actually should not matter in this case. Old Android phones didn't have an internal storage mount over USB (they primarily just acted as a USB card reader), which is why on older phones the files would have to be on an SD card, but that shouldn't matter...it really is probably more a matter of whether or not the car's stereo can read the phone in MTP mode vs "removable storage" mode...some can, some cannot.
Here's a pretty decent write-up that explains the difference between traditional mass storage mode and MTP, and more importantly why Android is now using it...
Ice Cream Sandwich explained: MTP - what is it, why use it, and how to set it up | Android Central