USB ports on modern computers cannot be manipulated directly in the manner that you are describing, because the processor is not accessing those wires directly. There is a dedicated host controller chip that does most of the job, usually one implementing one of the various Host Controller Interface specifications. That chip handles the low-level handshaking with the various devices and may just refuse to work with a port that does not have a proper USB device connected.
The main CPU accesses the USB port by programming the host controller, which imposes several restrictions on what can and cannot be done. In addition, all major operating systems that I am aware off have a pretty rigid interface for accessing USB ports. Even if the HCI allowed direct signal manipulation of the USB port, the OS interface does not.
Finally, wiring random non-USB-compliant hardware to the USB port is a very good way to fry the port and possibly the computer. USB is intended to be implemented by both the computer and the device.
If you want to control hardware with the USB port, then you could either use an adapter to another port interface (e.g. a parallel port) or incorporate a microcontroller that supports USB connectivity in your car controller.