Mine is wonky too on the 82 TD. It will come on when you push the button on the center stack, it will come on when you first open the hatch but go out when the hatch is fully-open. It can then be turned on by pulling on the interior latch handle.

I figure that its a switch or loose connection somewhere, but it hasn't been a priority.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)