I've been using NRGZ28's EnergyROM on my TP for a while, but the recent ones haven't worked with my in-car stereo (it works as a handsfree via BT).
Well, after a bit more fiddling today, I finally sorted it I think, so I will post the solution here for others, as it may be useful...
I did searching in the registry for Handsfree, and found the relevant key. I then did some googling and came up with this link http://msdn.microsoft.com/en-us/library/ms880945.aspx
I noticed two things: I was missing the NoHandsFree DWORD, and my Capability DWORD was a different value to the default.
So I added the NoHandsFree value, set it to 0, and then edited the Capability one to be 0x21, rebooted, and it works!