As with many things that have o-rings in aluminum bores, I have also found PB Blaster and patience to be effective. Enough lube, moving the part up and down eventually works the penetrant in. Problem for me on this one is that it was under the heater box and I was in a rush... make that impatient. Knowing that I would have access to the "other end" of the plugs it was not worth the effort. Oh I might have saved the harness...
Also noticing more corrosion in those bores nowadays too.