how do i change the cord on oster classic 76?

I ordered a new cord for the Oster Classic 76 clippers. I unscrewed the bottom of the clippers, unscrewed the black wire and the white wire..... now im stuck because it seems like the top of the old cord with the wires wont possibly come out of the hole on the underside of clippers. It looks like a thick square/rectangle piece of rubber about 1/2 an inch long thats connected to the cord wont let it come out. Ive had these clippers for about 5 yrs now, and Ive never had to replace the cord until now. What am i supposed to do? Also, once you do get it out, how to you put the new cord in? Thanks-