When you plug it in, it gives you the option to choose which type of connection you want. If you choose Charge only, it will show you your SD card.

If it doesn't give you the choice go into settings and clear the default.

 

Chances are the car charger requires you to plug in your USB cable from your charger somewhere? correct?

Either way, both of the incredibles I had could recognize when it was the USB cable when plugged into my car charger (recently had to get a new one, after I discovered this problem)... I had to do this because my standard verizon one wouldn't work, so I used to USB outlet up top to charge it while I drove.

For the record, I had the same exact problems as you, and it was MADDENING. It wasn't until I got my new incredible that I realized it was the USB cable.

Use the standard plug for the car charger... or if you're doing something kind of crazy and have a USB outlet in your car... go out and invest in a charger you can plug into a power outlet in your car. Not expensive.