Need a controller for a project car

I'm looking for a controller that could handle forward, reverse, left and right steering for a project car. Please let me know. We are looking for something that we can just hook up a controller to.

We have many customers that have done exactly that with our controllers, using joysticks or radio or an onboard computer, and sometimes a combination of all. See also the DARPA GRAND CHALLENGE web site. Several teams are our customers.

Is there anyway we can get the specs of such units. Such as controller interfaces and voltage allowances. Also we want to know is the father the controller is pushed the voltage changes to increase/decrease speed.

I assume you want to control a gasoline car.
Right/Left control is mostly done with a custom jumbo servo consisting of a motor (we have seen between 1/4 and 1/2 HP depending on the mechanics of the steering and the kind of soil, hard pavement being easier than grass soil) and a pot which provides DC voltage feedback to the controller.
The standard is 0 to + 5 Volt, where 0=extreme left / +5V=extreme right and +2.5V=center. Typically the total angle is some 18-0 degrees and sometimes a multiple turn pot is used, although not strictly necessary. The servo has some extra protection at the +90 degree and -90 degree positions (example stop switches). The controller is setup for closed loop position mode. The driving mode depends on the application. In local applications (drive by wire car) the driver sits on the vehicle and drives via a joystick. In Remote applications, like movie making etc, the driver is off the car and drives at sight via a radio (PCM preferred).
In fully autonomous applications the vehicle is driven by a computer (navigation computer) equipped with various sensors. Depending on your application, the controller will be set on the appropriate input control mode.
The forward/reverse can be as simple as driving a mechanical lever using some sort of electrical actuator. Throttle control is linear and is best obtained with a linear actuator connected to the throttle pedal, although a servo will do as well.