According to the devs, the Highwayman was originally supposed to have an entire fusion reactor lodged in the back, to explain why it didn't need fuel. Later they decided to make it refuelable with energy ammo and give you a trunk for storing stuff instead, so the reactor was scrapped, but the two boxes there are remainders from the original graphics.
"The car was originally going to be powered by fusion or fission or something nifty and nuclear (something that would allow us not to have to worry about running out of gas) and those big tanks in the back were representative of this big bulky power-source that was bolted on to the back of a '57 Chevy.
We had thought, at one starry-eyed point, that the car would be something that you could upgrade with more speed, armor, weapons, etc. and that it could be a more important part of the game—well, the engine and programmer tasking weren't up to the idea of having a moving vehicle (madness, I know) in the game so all of that went out the window. The power source was replaced by a much more mundane 22 cubic feet of cargo-space (more than any non-wagon vehicle in its class!).
So basically, the Highwayman's powerplant was downsized in order to meet with People's State of California emissions standards. It's often the case that a prototype car isn't exactly the same when it goes into production. "
-Matt Norton, Fallout 2 lead designer.
Wow, that's interesting! I never gave much thougth as to Highwayman appearance, ar what is for what - there is a sizeable trunk, so I don't have to carry all my stuff myself, and it moves fast around the world map.
Now, come to think of it, those two back thingies coul be easily fitted with stationary rocket racks, but I guess it would make the game more like MechWarrior, which it definitely isn't
The idea of weaponizing the car isn't that far-fetched. Imagine you starting in a random encounter with two raiders already dead on the ground - because you drove over them!