How can I repair a tear in a seat cushion in a new truck?

Take it to an upholstery shop. You may be surprised how inexpenaive it is and how great a job they can do.

Hi Rebecca, not sure what fabric your truck seats are covered in, but if it's leather, this tutorial will help - https://www.wikihow.com/Repair-Leather-Car-Seats

Use a curved upholstery needle and sew it. You can get one at Joanne Fabrics in the needle section.

Fabric, vinyl or leather?

A well disguised repair would be beneficial.