connectors at the injectors that had a U shaped wire latch that is not that easy to remove. If that is the case use a sharp pick tool to pull out the U part way. To replace just put U all the way in and push on till snap. Careful old connectors can crack.
If new style there is a tang to press to open the connector. In either case check for corrosion in connector and in the injector. Clean all corrosion and fill the connector wit dielectric grease before installing.
Note also corrosion can be several inches up in the wires at the connectors. Not easy to detect unless cutting wires and replacing connectors. This happened to me and I had to cut out about 2 inches of wire before attaching the new type connectors.