I just went through the whiplash scenario but it wasn't as severe as your's sounds. We got rearended in my vehicle.
For most soft tissue injuries alternating heat then ice has worked well for me. I always use ice for the first few days to keep the swelling under control. After that, I use heat for 20 minutes or so then ice for 10 minutes and try to do the routine as often as I can.
I also went to the chiro a couple times a week, just to get everything back where it should be.
Fortunately, my neck was only sore for about a week and somewhat weak for a couple more weeks. Now I'm pretty much back to my normal self except for some stiffness that is sticking around. Chiro and heat take care of that though.
Good luck, hope you heal up quickly.