At 9 months old, she's still rear-facing. Safety-wise, you want to keep her rear-facing (RF) for as long as possible, too (at least until the age of 2, but preferably beyond 2 if she still fits RFing height and weight wise in her car seat). She's 5 times safer RFing.
Are you looking at any particular kind of truck? Or just trucks, in genereal, with no specifics in mind?
To answer your question, in a regular-cab truck (that without a back seat of any sort), yes, you most certainly can place a car seat in the passenger seat, preferably rear-facing (for as long as possible), if it is not equipped with airbags OR there is a *manual* turn-off key/switch to disable the passenger side airbag. In this scenario, you'd want to keep her rear-facing for as long as possible because, yes, it's safer, but also because in that seating position there would be no top tether anchor for a forward-facing car seat. And you want to top tether all FF seats, if at all possible (it reduces head excursion/forward movement). It sounds like you know this already, but just for good measure to those who may search later, you should NEVER place a RF child in front of an airbag.
In an extended-cab, you can place any child car seat in the back that will properly install (in accordance with the child seat manual AND vehicle manual). For the bulk of car seats (and may also factor in with the vehicle manufacturer), that means having enough room for your RF install not to touch/brace the front seats and also enough vehicle seat bottom to have at least 80% of the car seat in contact with the vehicle seat after installation. It's definitely not impossible - people do it successfully all the time. It's just the matter of WHAT truck and WHAT car seat, know what I mean?
Hope that helps a little.