Are you certain it's from the rear crank seal , when you changed the seal did you replace all the other gaskets at back of engine , there is a gasket for the cam shaft cover, as its above the crank seal casing if leaking could drip onto crank seal giving impression of crank seal leaking. Also there is a gasket on the horse shoe plate that the crank seal fits into, also when removing this plate the back of the sump gasket will be disturbed. From past experience all the above gaskets would be changed when doing crank seal . If the sump gasket wasn't damaged when rear crank seal plate removed you could get away with putting a decent gasket sealer on plate where fitted to sump gasket. Because of work involved in doing all this generally would have reesealed all front of gearbox as well to try and eliminate all areas that could leak. In reality there tends to be some form of leak from a car of this age, they leaked even when they were fairly new.