It is alright to put a baby's car seat in a two-seater truck without a back seat as long as it is in the correct position for the child's height and weight. The seat has to be age-appropriate and buckled up.  However, you must deactivate the airbags and maybe push the seat as far back as you can because if a child is under 4 foot 9 they can get neck and face lesions from the airbags even in a small crash at low speed. The front seat is less safe, it is never safe for babies of any given age. But there's no 100% safe place in the car. The only safest place inside a moving car is in a car that is not crushed.