Actually, the A/C drainage line tends to disintegrate. It is just a foam tube that runs from the very center of the dash to and through a hole in the peak of the transmission hump under the dash. When its gone your A/C condensation drips down onto the floorboard.

Ken 300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles