Yeah mine had a plastic plug in it as well, with some sealant over it. Ended up using a wooden dowel to pop it out (it released from the engine bay and it's impossible to reach), drilled a hole in it just smaller than the 8ga wire I was using, and popped it back in. A little RTV on the wire, once it was run, and it's nice and water tight again.
As for running the wire in the engine bay, I found the best method is to straighten out a wire hanger, run it down through the hole, tape the wire to the hanger, and slowly pull it back through. Made it easy.