My z120bt does not seem like it fits in my dash. Do these trucks require trimming the plastic behind the unit?

yes almost all chevys have a piece of plastic that the back of the factory radio rest into, and most aftermarket radios are a little deeper than the factory, so i always cut that plastic panel out to make more room for all the extra wiring and stuff that the factory radio doesn't have, just watch what you cut, make sure that it doesn't support any of the dash trim, or that there is not any wires right behind where you are cutting.

i have a 09 Trailbazer SS and you have to trim the oem radio brace to fit aftermarket radio's . score the lower edge a few times and it will snap off.