SOLIDWORKS - How to make this assembly fully defined?

I have three parts - rolling_chassis_ln356.sldasm, body_main_ln356.sldprt and nose_cone_ln356.sldprt .
the rolling chassis is purposefully underdefined since the wheels are not locked to rotate. But when I assemble and mate the three parts together to form carriage_ln356.sldasm, both the other parts also show as under defined.
Can anyone please tell me exactly how to make these fully defined? (not including the rolling chassis; all files attached)