This happened to me. If it is not the speedo gear O-ring, then it is likely the case halves where they meet. The ring gear flings trans fluid up to this location (ports inside take the flung fluid and use it to do a low pressure feed of the bearings).
Clean the case halves on top using brake cleaner. Take it for a highway drive (higher trans "pressure"), then check to see where the leak is (more towards the firewall than the bellhousing). If its leaking there clean it it again with brake cleaner and rags, get it very clean. Use ~ 240 grit (or a little coarser) to sand both halves. Then carefully spray two coats of Permatex gasket aerosol coating on (wait for the first coat to get very tacky first). Will try to get the exact product name off of the can tonight.
This leak appeared for me after I had a professional shop install my Quaife into my trans which was out for clutch replacement, then the leak appeared after a dyno session (7000 rpm in 4th gear = lots of trans pressure). Was able to limp it along using the technique above for roughly 80 K miles until it was finally time to replace the clutch again, and install my NESHO Gen III subframe conversion. The second time I split the case myself, carefully cleaned off the mating surfaces and then resealed using anaerobic sealant. Presto, no more leaks, even after hitting 110 at NHIS and somewhat higher on the old course at Lime Rock.
