How do you replace a 1984 Ford Mustang LX headlight bulb?

You can't replace just the "bulb" as this model still used sealed beam headlights. You have to replace the entiredamaged or blown headlight assembly (note that these Mustangs had 4 separate sealed beam headlights and when I refer to the "entire assembly" I refer to one of the 4. Also note that 2 of these light assemblies were for high beams and 2 were for low beams. Be sure your replacement light is the correct one). Below are directions for removal/instillation 1. Remove negative/ground (usually black coated) battery cable. 2. Remove the headlight bezel (little square metal piece around light assembly) by removing the bezel retaining screws and slipping it off. (Some models may have retaining springs instead of screws, simply pull these off using a pair of neddle-nose pliers). 3. Remove the headlight retaining screws. (Be sure you loosen the retaining screws, not the adjusting screws [these are usually noticeably larger and are the screws holding the actual headlight frame in it's accurate position]) 4. Gently pull the headlight out of it's holding frame. 5. Disconnect the wire connector from the back of the headlight and remove the light assembly. Installation is simply steps 5-1 (reversal of removal) You may need to have the lights professionally adjusted, but if they were properly aimed before removal, and you didn't disturb the adjustment screws, they should be properly aligned. Hope this helps.