Like Andrew said, push the pins all the way through on the cowl vents and then slide them towards the front of the car. There is a little tab you don't want to break. I used a small nail punch and a small hammer to tap them out. The closer they get to going all the way through the lighter you want to tap so they don't disappear.
The square pins on both cowl vents towards the middle of the hood are the hard ones to find. When you push them through, they fall in to the drain tray for the wiper arms. If they are not at the top where you can see them, chances are they fell all the way through and are stuck where the water drains in to the engine compartment. There is a fat rubber hose, on each side, that comes out of the fire wall and are semi-pinched at the end. BE CAREFUL...that's where my little plastic pins wound up.

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"