I just hate Rhino wires, you can't see **** on them
The model is looking extremely nice btw!
As for that setup, it's rather easy. Have a null for each front wheel, and one null for the rearwheels (Wheel_Left_Pivot, Rear_Pivot, whatever). So three nulls in total sofar. Parent them a Null (Wheels_Null). Also add a null to control the rotations (Wheels_Controls), parent this one to the Wheels_Null too. Then make sure the Nulls for the wheels (those three) have Wheels_Null as their Goal Object (select the null, press m, etc).
Add another null for the whole car (Car_Null), and make a null for the whole body (Body_Null). Parent both the Wheels_Null and Body_Null to Car_Null. This way you can move the whole car with the Car_Null and the body can independently move so you can mimmick suspension movements.
Then all you got to do is expressions. It sounds hard, but it's rather simple.
For instance, to make the rear wheels spin when rotating the Wheels_Controls, you have to select Rear_Pivot, open up the Graph Editor, select Rear_Pivot.Rotation.P, go to the Expressions tab, make a new expression and call it "Drive" or something. Then just put
-1*[Wheels_Control.Rotation.P] as the value. This means that the Rear_Pivot Pitch is controlled by the oposite of the Wheels_Control pitch (this is just how I liked it to be). Same goes for the Wheel_Left_Pivot, etc. But then also make a new expression to steer,
1*[Wheels_Control.Rotation.H], etc. Play some with it, it's pretty easy
To add something, I've set my Wheels_Controls H rotation to a limit of -30 to 30 degrees, just to make sure whatever I do the wheels don't steer more than 30 degrees.
Good luck, or... I could just send you my scene, lol. But it's fun to do it yourself