theres a kit you buy. its like a space type thing. if you look at this picture. around the head unit theres this black thing and you out in to take the full space of the old head unit. once you put that in theres this piece of metal thats shaped for your new headunit. put that on the new black space taker uper. and then your head unit should just slide right in with the face plate off.
im using a kenwood head unit not a pioneer so it may be different but they are preety much the same company to be honest and ill look up a kit to fit your truck. i just need the model number of the head unit and the year of your truck