Spanish police inspected the vehicle after noticing the driver seemed anxious and found the man crushed between the engine and radiator, suffering from severe dehydration and lack of oxygen.
The vehicle was crossing into the Spanish territory of Ceuta from Morocco.
A second man was later found concealed under the backseats of the vehicle with similar symptoms.
Both men, thought to be from Guinea, were taken to hospital for treatment and police arrested two Moroccan nationals who were driving the vehicle.
The discovery follows a string of migrant deaths across Europe as thousands attempt to flee across the Mediterranean.