In the past, when I was using GPS and tethering, I would lose charge even when plugged in. The problem was that the charger looked like a USB device instead of an AC device and was limiting the charge rate.
Try plugging the phone into the car, and go into Settings, About Phone, Status. The Battery Status should read "Charging (AC)" for the fastest charging. If it reads "Charging (USB)", it will only charge at some reduced rate, maybe 50%. If it doesn't show that it's charging at all, it's something else going on.
I eventually found this HTC charger that the Optimus sees as an AC device:
Amazon.com: HTC MyTouch Dual USB Car Port/Charger: Cell Phones & Accessories
Hopefully, this will be your fix. If not, maybe it will help someone else.
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