Rear Oil/Fluid Leak?

[acpilot]

I am hoping someone can shed some light on a leak that has developed on my 94 Cabriolet. I just noticed it the other day while checking on my car in storage. A small wet/oily spot under the rear of the car almost dead center under the trunk. It seems to be coming from a black round plastic access of some type. It is well past the rear suspension area so I'm not sure what it is for. Due to fact it is in storage I was unable to investigate further.
Hoping someone can tell me what I was looking at and possible cause.

Thanks

[Ruffiano]

only thing back there that could leak is the hydraulic fluid for the automatic roof