Grand Theft Auto: San Andreas/Intensive Care

Drive to the hospital. You'll find that Johnny's been taken out. There are 3 ambulances driving in Las Venturas. 2 of them are the real deal, one of them is a Mafia truck. Find out which one is the right one by slamming into one and see how the driver reacts. When you find the right one, chase it, get the driver out, and drive the ambulance to the checkpoint.