This is what I got out of what you said:
Working on 82 stitches to start with.
Body work to 11 inches. Join. Knit front 5 and back 5 stitches together and then work in the round until piece is 18 inches.
Making the car seat belt hole:
Knit to the middle of front, turn
Purl to middle of back, turn
Work back and forth on those stitches until you have 19 ½ inches.
The pattern would be nice to see if you can provide a link or something. But if you can't we'll need more information to be able to help.
Have you made the front and back in two separate pieces up till now or are you possibly working all 82 stitches of front and back on a circular needle but working back and forth flat?
I don't get the part about
Knit front 5 and back 5 stitches together
I don't see how that would work into anything you have going. But maybe if I understood the construction of the whole works better I'd get it.
Where does the seat belt hole go? On the chest area somewhere? Try to describe how this is made and what you have done so far.