In that case you can register a pick-up in the 'passenger' class if it meets certain conditions, depending on its unladen weight.
Does your modified or unmodified pick-up truck have an unladen weight less than 6,001 lbs?
Yes, that is the case.
Then you can register it in the 'passenger' class if the pick-up truck does not have any business advertisements and it is used exclusively for non-commercial purposes.