My trunk.
But I have a friend who bought one of those rod holders made for the inside of a boat cabin...well he put it in the inside of his jeep on the ceiling.
This here thing is pretty cool........
http://www.tackletour.com/reviewthulecastaway.html
Also there was a time when my pole wouldn't break in half....I connected the two pieces together and couldn't get them apart no matter what I tried. It just so happened I had a ton of junk in my trunk (Like J-LO) so I was driving around with a 12 foot surf caster going from my back seat out of the front passenger window....