I have a leaky heater core on mt 64SS Impala without A/C. My question is how do the dash mat retaining plugs come out on the inside. There seems to be a hole down the center of them but no screw that I can find. Do you just pull them out? I don't want to wreck the dash mat because it's in perfect condition. I looked in the factory shop manual but it just says remove the plugs but not how to. Thanks for any help.