This is a common problem.
It bugged me sufficiently that I investigated it in detail.
The Desire, and presumably some (all?) other HTC phones, employ relatively complex charging circuitry.
When you plug a USB cable into the phone, the phone does at least two different checks to determine what type of power source you have just connected.
If you have plugged in a mains powered official HTC charger, which has a rated output of 1A, then the phone knows that it is safe to draw a maximum of 1A from that charger.
The phone will then draw enough current to power itself and, on top of that, charge the battery at the same time.
This current will typically be in the region of 800mA (0.8A) to 900mA (0.9A).
Under these conditions there is enough current to power all the functions of the phone, including WiFi, Bluetooth and GPS, as well as the usual GSM radio and the phone's other functions, as well as being able to charge the battery.
However, if the phone believes that it is connected to a power source with a lower rating such as a standard USB port, then it will limit the maximum current that it draws from that power source to between 400mA (0.4A) and 500mA (0.5A) as this is the maximum officially provided by a USB port.
In other words, the phone is intelligent enough not to overload a standard USB port but, when more power is available, it is able to use it.
The mechanism that HTC uses to detect a power supply capable of supplying 1A, as opposed to a USB port, is very simple indeed.
When the phone detects that an external power source has been connected, it checks to see if the two data lines of the USB connector on the bottom of the phone have been short-circuited.
If they have been short-circuited, the phone takes this to mean that a suitable power source has been connected providing a current of at least 1A.
If the data lines are not short-circuited, the phone assumes that the power is coming from a USB port or other device not capable of providing more than 500mA.
In practice, the way this has been implemented is that within the official mains powered HTC charger, the two data pins of the USB connector are shorted together.
As soon as you connect this charger to the Desire, the phone detects the short-circuit and knows that it is connected to a charger capable of supplying 1A.
This particular trick seems to be something unique to HTC rather than being a universal standard, although this is a bit of a guess on my part based on having looked at only a few other chargers.
What this means is that if you have a car charger that is rated at 1A or higher, your HTC Desire will still only draw a maximum of 500mA from this charger.
This problem is easily rectified by opening up the charger and soldering together the two centre pens of the USB connector so that the phone sees this short-circuit and realises that it can safely draw I higher current from the charger.
Unless you know what you are doing and fully understand what I have explained above, then please don't go fiddling around with your charger.
I have carried out this modification myself on a couple of non-HTC mains-powered chargers and a couple of 12V car chargers with 100% success.
I have, however, found that some 12V chargers, even though they are rated at 1A or even 1.5A do not result in the Desire drawing the expected current.
What I found was that the phone would draw only about 250mA and then, after I had shorted the data terminals within the charger, the phone would draw about 450mA but not the 850mA or so that I had expected.
I have yet to determine with certainty why this is but it appears that as the phone begins to draw current from the charger it is able to detect if there is even a relatively small dip in the voltage coming from the charger and, if so, the phone backs off on the amount of current that it draws.
I will be doing a few more tests in my electronics lab to try and get to the bottom of this and provide a more detailed analysis and, hopefully, a useful solution.
In the meantime though, I have at least solved the problem that I was having and, based on numerous forum posts, the same problem that many other people have been having with car chargers not effectively charging the Desire.