There is only one way you can use your cars from your garage. I'm going to blunt like I am at work...You can only use your cars from the garage in 2 player mode only. When you do the traction control system (TCS) is turned off and the car reverts to a default car, meaning no tuning nothing just the way it was when you first bought or won the car, you cannot see the car until you have chosen the car to drive against a firend or opponent.
Unfortunately GT4 hasn't given us the opportunity ton have this feature like it was when GT1 & GT2 came out.
that is the answer to the question...
Or you can use your garage cars by plugging in another PS2 controller or wheel, select 2player mode in Arcade mode, go to top and select your position, 1st player or 2nd player, pick the garage option and chose the car. This way you can drive by yourself in Arcade mode using your hard earned cars. but yet they still go back to the way the car was when you first got it before tuning it or by wheels and stuff ( includes the spoiler )
hope this helpt