I thought I'd share with you all how I set it up in the car. Hope it is helpful for someone out there
I have used Hydrapaks in the past however there are other brands such as the more well known Camelback.
This is the straw in position
As you can see, I have the straw mounted on the seat belt. Usually on the packs you have a locating strap wrapped around part of the shoulder strap to locate the drinking straw for either easy access or so that you can get to it with your mouth
As I had a second old pack lying around I chose to pinch its locating strap so that I could fit around the seat belt.
I joined them together as follows
Then completed the link
Of course the link goes around the seat belt.
Mounting the backpack was pretty simple. Pull the headrest off and thread the shoulder straps around the headrest posts
Then stuff it into the netting behind the seat. You can set the headrest a bit lower if you want too
Note, I installed the backpack backwards in case I wanted to refil it

