Download this rar and extract it out to a folder. it should be all you need.
Click the cmd icon and that will open up a command window. Reboot your phone into bootloader and connect it to your PC. Make sure on your phone says FASTBOOT USB.
Next type: fastboot oem rebootRUU
Your phone screen should go black and show the HTC logo.
Next type: fastboot flash zip firmware.zip
You should start to see it updating the radio in your command line window. Your phone will show a green progress bar but just ignore it. It will not show its completely finished. The flash shouldn't take too long and once its finished and you are back to the command line prompt type
fastboot reboot
As with everything else, the disclaimer always applies. I've added a MD5 file to check to make sure the firmware zip is good but always check. I made the firmware.zip only containing the radio and it worked for me, but always check the MD5.
https://www.dropbox.com/s/e3sskmseolpsn7o/fastboot_1.29radio.rar