Here is a short Kellogg's commercial featuring the Monkeemobile. Click picture for video..notice the trailer hitch!

Some other shots showing the trailer hitch

Screen grabs

and the trailer itself
larger
"Dean also told us he built a trailer for the car from four GTO rear quarters, matching the design of the car. Along with serving as a stage, the roof of the trailer would slide off, allowing the Monkees to store their instruments." (MUSCLECAR REVIEW July '68)