I'm wondering if there is some sort of script or plugin that will make vehicles seat as many people as there are seats instead of one person per door. For example, the cab of the fire truck has (I think) 7 seats, but only 4 doors on it, so only 4 people can sit inside. Is it possible to utilize all 7 seats?

Definitely possible. Check out this:

 

Im not sure, but would looking at the insurgent vehicle meta section help? Because that vehicle you can fit three people in the back seat. Just a suggestion.