but I wanted to control via USB...
If it connects through a USB port, the iPod is only acting as a storage device, which the car audio system accesses to read the song files. The car audio system is in control and the car audio system (not the iPod) is playing the songs. It's just like connecting a USB flash drive with song files on it.
If you happen to have a USB flash drive, put some song files on it and see if that works with your car audio system. If that works, you may want to consider getting a 16GB USB flash drive for about $10-15, and using it instead of an iPod. Here's one that barely sticks out of the USB port.
http://eshop.macsales.com/item/SanDisk/SDCZ33016G/
Then, you can just leave it connected to that port, and only remove it to update the songs. No docking cable to get in the way. You don't need to constantly plug and unplug your iPod (which causes wear and tear), and you don't have to worry about forgetting your iPod in the car, which can cause damage from extreme heat or cold, and become the target of theft.