No plugs on the back of the engine. The rear main seal also seals a coolant passage, so if the leak if from there, the rear main is the culprit. Make sure the coolant pipe seals are not leaking as they will drain out the back of the engine.
Here is a back of the engine pic, you can see some of the coolant pipes. The intake manifold needs to be removed to replace all of the seals. I just did mine recently after the shifting of the pipes during trans removal caused leaks.