How do the heater vents go together

[86chevyk20]

I have an 86 k20 bought new vents and those plastic peices that bolt to the dash pad. How the hell do the adjustable vents to lock in the plastic holders?????? There are not any holes in the plastic for the vent to snap into

 

[89Suburban]

There should be little divots/recessed holes in the housing on the sides, you can barely see them here:

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[86chevyk20]

i dont have the wholes at all. none! brand new from LMC.. maybe i have to drill my own