As someone who was an EMT and ambulance crew for 15+ years on a fairly busy crew, you never want someone riding along with in the ambulance in these situations. Someone upfront is tolerable, in the back is not ideal.
I was going to say I didn't think they let you ride in back. But then remembered Mom rode upfront when I went as a kid but I did ride with my baby. He was fairly stable though so I imagine that played in.