In order to replace the ignition key cylinder you must remove the housing assembly from the dash by using a dash bezel tool available from Thunderbird parts dealers. Disconnect the battery before removing the cylinder housing. Once the housing is released it can be dropped below the dash. In most cases the wiring can remain connected.
Photo #1- Once the switch assembly is below the dash you will see a small pin protruding throw a slot in the housing.
Insert the key into the cylinder, push down on the pin it may be necessary to turn the key to accessory or on position before pushing the pin down.
Photo # 2- The small pin should be pressed down flush to remove the key cylinder by pulling the cylinder out of the housing.
Photo # 3-If the key is missing it may be necessary to remove the switch cylinder housing from the dash. After removing the housing take a small punch and drive the small pin down into the cylinder, then the cylinder can be pulled out of the housing.
Insert the new cylinder into the housing with the pin pushed flush, insuring the pin aligns to the slot in the housing. When the key is turned to the on or off position the small pin should pop up into the slot which locks the cylinder into the housing as shown in Photo # 1.
If it was necessary to remove the wiring , reconnect and reinstall the switch housing back into the lower dash, you are good to go.