madclown nuclear extensión changes the nuclear fuel to another tech, wich is not a prerequisite for the car mk3, and since the car needs nuclear fuel to be Crafted (and researched), it ta<kes it as is one tech was missing, and you cannot research it (just the MK3)
If you plan on lettin it not craftable with madclown nuclear, could you remove it from the tech tree?, its annoying having techs in your tree you cannot research