"Black plastic trans cover"...please post a pic so we can know exactly where you are seeing this leak...
Color of the oil leaking...red is auto trans oil...brown or black...engine oil.
If it is trans oil, it can be leaking from a couple of areas...
pump seal,
pump gasket and O ring, pump to case bolts (these should have a rubber center thin shim washer style gasket on each bolt,
convertor weld area small pin hole
All above require trans removal to correct
Could trans filler tube O ring or seal or modulator or pan gasket
If it is engine oil it could be rear main seal, sump pan gasket, oil pressure switch, manifold seals, or rocker cover gaskets.
A pic of the leak area will help us help you...
