How unplug wiring harness at firewall

[hsi111]

Hi in there. I just bought a nice 1997 tahoe 4x4 350 with a blown head gasket. I am tackling the job myself and have disconnected everything on the motor with no problem. Then came what should have been the easiest part; the harness to firewall. I have spread the two plastic clips and beat on the plug (down) for an hour and it won't budge. Am I missing something here or is it just stuck? Thanks