This small tray-back cargo truck was reportedly fitted with the gun turret off a Russian war machine called a Boyevaya Mashina Pekhoty (BMP) by the Free Syrian Army. As you can see in this video, it seems to actually work.
The truck's suspension gets a bit of a wobble going when the weapon's fired, but the vehicle looks like it's bearing the load well enough otherwise. This would certainly have more speed and mobility than an armored tank, and could be concealed to blend into traffic a lot more easily than traditional armor. With a canvas over the back, of course.
Military Studies says this clip of the improvised mobile cannon in action was captured in Sheikh Miskin, a town in the south of Syria.
Hat tip to Pentimento!