When you state that they will "pair but not connect" does this refer to incoming calls, outgoing ones, or both?

Unfortunately ALL current HTC's have a very crippled Bluetooth chipset fitted and this may be the cause of your problem. For example, none of them (Windows Mobile included) are capable of handsfree Bluetooth voice dialling and this is particularly bad given that WinMo is very strongly featured with voice commands and many HTC models from about three/four years ago had these features as standard. The crippled chipset also means that until very recently, most HTC models could not use Bluetooth to transfer files of any kind, but some models such as the Hero have recently had an update to allow limited functionality in this respect.

I can't state with certainty that the problem you have is to do with the poor Bluetooth chipset which HTC currently appear to be contractually obliged to use, but it's a definite possibility. What is definite is that it's not an Android issue (although Android current and planned releases do not yet incorporate Bluetooth voice dialling either, but that's not an issue when you're using the car to handle the calls).

If you can add details of the exact make and model of your vehicle then I'll try to look into this further for you: it's also not uncommon for vehicle manufacturers to install poorly chosen Bluetooth chipsets and sometimes the cause is a simple case of Brand A simply being unable to get along with Brand B. If you're in a real hurry to resolve this, then definitely change your handset, but insist on being able to try the replacement out with your car before committing to it.

Across the board, Bluetooth on many phones seems to have regressed and is poorer than it was a few years ago. Even the i-Phone is very poor except for the flagship 3GS but even that model is *very* picky about which brands it will usefully work with. Currently, the best and most reliable Bluetooth in my opinion and experience is in the Blackberry range of handsets. If you don't want one covered in buttons then go for the Storm 2 touchscreen. It's not as fun a gadgety as the HTC and Apple models, but is a good solid performer and does everything a phone/mobile browser needs to do very competently.