aggressive dog

I live in California and I have an aggressive dog. He is small, about 40lbs., and he looks like a nice dog. However, when he is in the back of the truck he feels that it is his area to protect. He has never had any formal training except obediance (i.e. he is not "attack trained"). I tie him into the truck when I leave it parked because if someone walks too close to it he will lunge and attempt to bite. The truck is a large one-ton dually, and there is no way this dog could bite someone unless they actually reached into the vehicle. My first question is this: if someone reaches into the vehicle and the dog bites them am I liable even though the dog is tied inside the back? My second question is this: Can I put magnetic signs on the vehicle to warn people that this "cute" dog is not to be petted? And if so, what should the exact wording be? A friend told me that if I put up something saying "attack dog" I am liable because I knowingly left a trained attack animal unattended. I was thinking somewhere on the order of "Dog in Vehicle: Do not approach". Please help!!
I will not put this dog down. He is not aggressive unless he is on a leash or in the truck. He is my pet and constant companion and I bred him so I've had him since birth. He is in all other respects the perfect dog. He is only aggressive because he feels it necessary to protect his/my property. He is not a mean animal and is good around children.
Thank you for any advice,
Jessica

JessicaS, I understand your feelings about your dog, and am in agreement with IAAL's answers. However, I have to ask.. if you are so careful about your dog, why do you haul him around in the back of your truck?? Why not let him ride in the cab, or at least leave him home. After all, you wouldn't like to see what happens to a dog that is tied in the back of the truck during an accident... of if the dog bounces or jumps out of the bed while moving.

Bottom line... protect your pet.. leave him home or let him ride inside the cab/car.