Is the car considered the property of the maternal uncle and his heirs after him, if it was registered in his name as a condition for the father to purchase the car from the Armed Forces exhibitions at a cheaper price, knowing that the father is the actual buyer of the car?

If the sale is free from deceit and fraud, there is no harm in it. The fact that the car is registered in your uncle's name in official documents does not mean he owns it; rather, it belongs to your father. It should be transferred to your father's name, or its true ownership should be witnessed and documented to safeguard rights and formalize them.