From the engine bay side, maybe some WD-40, and lightly squeeze around the connector block with pliers or channel locks to loosen up the old, probably hardened up connector. Engine bay side connectors just plug in with no type of retainer clip. Interior side just pushes thru the fire wall, there are little metal clips cast into the connector that keeps it in the firewall, easy to push back with a small screwdriver. Individual wire connections could be corroded together.
Check all the wiring for burn thru, pinkie got HOT! Check for corrosion in the connectors. Dielectric grease when you plug them back together.