Not exactly sure how FM modulators wire up, but I am assuming from the way you describe it that it wires up to the speaker outputs? Of course you can do that. One thing to keep in mind is does your truck have a factory amp? That may make things tricky, I know my 94 Ford Probe GT I used to have had a factory amp. Your best bet is to try to find some sort of electrical manual for your truck. That may be hard since your truck is pretty new, but I am guessing they used the radio thats in your truck in other models too.

This may not be much help but not having much experience with FM modulators and not being able to see how it hooks up makes it hard for me to say much.