The problem you are having is that most OEM car charger manufacturers donít appreciate how our smartphones work, and they neglect to short the two data pins out in your charger.
When you plug your official HTC wall charger in, the phone detects that the two unnecessary data pins are shorted out (as obviously HTC know to do this) and the phone will not limit itís own charge to 500mA.
Use an unofficial charger, and the phone see no short on the data pins and so assumes itís plugged into a PC/laptop USB port, and will restrict itís own charging to 500mA to prevent damage to a motherboard.
Your car charger may very well be capable of dishing out 1A, but they manufacturers just didnít appreciate what they need to do to tell your phone to start taking it.
I have had this trouble before, and there are a couple of options.
1) Only use official HTC car charger.
2) Take a spare charging cable, and carefully peel back the insulation. Then carefully cut back the screen. Youíll find 4 wires. Red, black, green & white. It is the green and white that are data. Cut these. The end of the cable that plugs into the phone, twist the white and green cables together. Insulate with electrical tape. Tape the cable back up. NEVER USE THIS CABLE WITH A PC/LAPTOP. But you should now find you charge at AC.
Been using a modified cable in my car charger for years, including with this HOX with no problems.