Does it have a power distribution box under hood? Check dimmer switch and wiring
It is a relay or switch problem. A circuit breaker or fuse won't keep a light on.
If headlamps are not working but the parking lights are working can it be the fuse that is faulty
A block heater does not use a fuse on the vehicle It is like a heating element on an electric kettle , it has a circuit breaker or fuse on the electrical supply panel in the building ( or if it is a plug in post in a parking lot , the breaker is inside the post )
That's how 1964 1/2 thru 1967 Mustangs lights are designed to operate.. when the headlamps are turned on the amber parking lights are turned off. That was changed sometime in 1967-68 so that the amber parking lights remain on when the headlamps are on.. It is possible to install a later model (1968) type headlight switch and wiring if you really want the parking lights to remain on when the headlamps are on however. Electrical Wiring drawings are available (to purchase) for every year mustang from almost any mustang restoration shop/website.
You probably have a faulty headlight switch. Replace it and that should fix it. Been there, done that.
Should be parking/tailight circuit
If the headlights do not come on and the parking lights do in a 2000 Jeep Wrangler, it is possible that the fuse is blown for the headlamps. It is also possible for them to both be burned out due to touching the glass part when installing them. Oil in the fingers will overheat them and burn them out.
Should be the same circuit as parking/taillights
See Headlamps, Front Parking and Turn Signal Lamps on page 6-36 of your "Owner's Manual", under Section 6: Service and Appearance Care - Bulb Replacement.....
See Headlamps, Front Parking and Turn Signal Lamps, under Section 6: Service and Appearance Care - Bulb Replacement, on page 6-36 of your "Owner's Manual"...
See Headlamps, Front Turn Signal, Sidemarker, and Parking Lamps on page 5-48 of your "Owner's Manual, under Section 5: Service and Appearance Care - Bulb Replacement...
See Headlamps, Front Turn Signal, and Parking Lamps on page 5-53 of your "Owner's Manual", under Section 5: Service and Appearance Care - Bulb Replacement...