Rear Main Seal should pertain to the engine & not the trans. Take the car to another repair shop & have them look at it. If you have the V6, it could be the oil pan gasket that is leaking. Those are known to be leakers. If it is the rear main, the trans will have to be removed to get to it.
05 Copper 6S w/mods
87 Sapphire Blue RX7 TurboII w/mods too