Keep blowing a fuse on marker lights and dash lights (same fuse for both)

blowing fuse 4 ,F4, 20 amps? (not trailer fuse,marker)
it feeds the brown wire out of head/park switch
13 park lamps on BROWN

well the park feed is shorted clear as day.
only where it shorted matters
park lamps can be 12 lamps.
2 front, 2 read, side lamps
license plate lamp, rear
the seam park feed goes to the cab too.
seems dome is shorted
and lamps sure can short out and the wires
this old easy.
the online manual sticks no dome ckt shown
old trucks then had no digital manual
so all we see today are lamp weak and grossly incomplete scans.
buy a used real F150 manual off ebay OEM real.

Check the tail light bulbs & marker light bulbs.

Check the fuse. A bad bulb can cause a fuse to blow.

The tail and marker lights and Instrument lights are on the same circuit so both will go out if the fuse for this circuit blows. If fuse continues to fail, check the tail light or marker globes, including the globes for the number plate, for a globe that may have an internal short causing the fault and replace as necessary. As you have already checked the wiring near the light fittings for any chafed wires that could be shorting to the body causing the fault, then a closer inspection of the wires inside all the light assemblies for any shorts may be needed.

It could also be that the instrument dimmer control has failed and is causing the fault. The electronic bits inside the dimmer control for these lights sometimes go faulty. Try disconnecting the connecting plug on the rear of the dimmer control and checking if fuse still blows. If fuse continues to fail, then the fault in the tail and marker lights circuit.

Remove all the tail, number plate and running light globes from the light assemblies and leave the instrument dimmer control disconnected and check if fuse still blows.

Disconnect the plug near each of the light assemblies to see if any of the individual assemblies is causing the fault. With all light assemblies disconnected and fuse continues to fail, then the fault must be in the main wiring loom some where.

If you have a Multimeter, it could be used to determine which light assembly has a fault between the lamp supply and earth connection.

electrical dudes says stop puting fuses in it now/// over time more stuff gets injured by exsesive heat look for any thing new some one has installed eg . stereo check trunk for damage in the corners near tailights and front marker light

Are the bulbs out of the sockets and the fuse blows?
Take bulbs out and check again
Place one back at a time if fuse was OK with bulbs out to locate bad bulb.
Otherwise.......
Do this........if still needed with bulbs out and fuse blowing.
Put a circuit breaker in to replace the fuse of the same amperage.
It will trip off then back on.
Use a standard cheap magnetic compass........place along lighting wiring........the wire with current will cause the needle to jump as circuit breaker trips on and off.
Follow path of current to problem.....the compass will stop moving once past the shorted /grounded wire/connection.

Check your fuse for your tail lamps. GM runs the dash, park, side marker tail and clearance lamps through the same fuse. This way, if the fuse does blow, you know you have no lights, except head lights. Please read your owner's manual for your vehicle. It should tell you this.

you have a short somewhere, unplug the tail lamps one at a time and see if the fuse still blows. if you disconnect one and the fuse does not blow and everything else works, the problem is in the tail light assembly. if not, the short is most likely in the wiring harness which can be a bit tricky to diagnose. alldata.com/diy offers one year subscriptions to cars with all the repair info and they have excellent wiring schematics. dont forget to try the front marker lamps in addition to the tail lamps.

check each of the terminals of those bulb you mentioned. wonder if there's a relay associated to this circuit, check for short if this is the case. remove all the light bulbs [of course - the dash may not be necessary (it's a PITA) & let's hope that it's not the culprit]. visually inspect those bulbs & replace if something unusual. put back the relay if it has no short, turn on the switch, let's hope the dash lights up & it won't blow the fuse anymore, otherwise look for uninsulated wire touching the chasssis or ay metal part of the car. you may continue replacin the bulbs.

Took it to the shop. The side marker lights were missing on my son's car. The wiring loops from the side marker lights to the tail lights. As the wires were touching in the front, they shorted out the tail lights. Capped the wires, problem solved!
PS: ordered side marker lights, too.