If you remove the glovebox there is a vacuum manifold of sorts. Test the different lines with a mityvac or similar and see if you can actuate the vents and if these lines hold vacuum. If you have leaks that will be one reason the vents aren't working. This thread has all the information you need.

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1987 300TD 231,000
#22, afterglow, plastic fan, euro lights, alternator upgrade, cluster housing update