If this is the same as on my 81 300TDT then if you remove the seat back panel (one little screw in the bottom) then you can see that the top of the plastic cover is actually screwed in from the back.
Now that you have the seat back off you can easily remove the top screws from behind.

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Current: '91 300TE 4MATIC 317k and climbing...
Former:
'81 300TD Wagon 168K "Tank"
'83 240D 216K 4spd manual "Da Bear" (aka best car ever)

"Never sweat the petty things...
and never pet the sweaty things."