Usually the default passkey is 0000 or Sometime 9999. You will have to look in your HTC manual to verify.
I also think since HTC has included so much functionality in the ONE X to be used with it's new BT adapter it might be making this difficult. (Could be way off base here) Maybe check-out this video and see if "might" be a way for you to go. Good luck and hope you get connected.
MWC 2012_HTC One X Car Dock Demo (English) - YouTube

 

I have found this too. I have Bluetooth audio so I can talk hands free but can't seem to get the phone to find a connection. Any help would be appreciated.

 

You will need to make the phone discoverable. By default the Bluetooth is on, but the phone is not discoverable until activated.
Go to Settings>bluetooth. Then tap on the top line, which will be whatever your phone is called and it will change from only visible to paired devices to Visible to all nearby BT devices. This will time out, which you can change in the menu tab on that screen.

I had no problem setting mine up for the BT phone in my Mazda.

Think you may have got past this though. I'll leave it here for others though.
I used to sometimes get a problem with my old HTC on BT compatability with other phones. Might be that. Try a reboot of the phone.

 

Thanks for that. Will give it a go in the next couple of days and let you know how I go.

 

I read online somewhere that if you use your steering wheel buttons to connect, it works instead of using the buttons on the stereo. ...if your car has steering wheel bluetooth controls buttons. It happened to work in my car, though I have a HTC Desire.

Hopefully this helps and good luck!