Some vehicles are bigger and there is a "divider" between the front and back seats. Or you can get vans to kidnap people.
I rather like the idea of using these that already exist in-game and thus, encourage chars to own more than one vehicle, rather than having one vehicle with a trunk that may have multiple uses for it.
Just my two cents. Maybe I'm wrong. :d
Is there any appetite for this?
There are items in game for decluttering cars overall, but having trunks could also build off this. Maybe certain cars have a trunk, maybe others don't. Perhaps add an item limit to what can actually be kept in cars outside of trunks, to incetivise buying one with a trunk if you're running around with stuff. Even outside of it being a mini room, just make it function similar to a locker if that'd be easier to do.
Don’t want your topside rivals to know you’re saving up for an aero? Get the best security system, pull up somewhere semi secure and have yourself a trunk nap! Topside camping!