Well kinda mad but I was planning to going to South Lake Tahoe this morning but my taillights aren’t working. My lights activate when I use the turn signal/ hazards and brake lights. My headlights work. No fuse is out. I’m assuming it might be the switch. I just need some advice. I have a 1990 f150 4.9
update.
I have no power going to the rear taillight wire only wires that have power when activated are my hazards/ turn signals and brake lights. No power in the for the taillights. I’m planning to replace the headlight switch

 

The parking lights up front work, yes no ?

 

yes they work

 

License lamp yes or no?

 

no they don’t

 

There are 3 diagrams above, the 3rd one is for side marker check that if your side markers also are not working.
On the 1st diagram look at the license lamp circuit first, that's easier. Circuit 14 is the 12v feed (when parking lamp is turned on), circuit 57 is the ground, and G100 is the ground point. You might have a loose or dirty connector at G100, wire brush clean them real good first. Then voltmeter red to BR wire at the license lamp, voltmeter black to G100, any 12v or not? Next, same red to BR wire, but voltmeter black to another good ground source other than G100.
Not sure how many terminals are connected to G100, but cleaning G100 first is a good starting point, then follow circuit 14 and check for voltage, look at the red arrow above. Depends if you have rear bumper or not, check splice S416, S417, connector C401. Use contact cleaner (or 91% isopropyl alcohol) to clean.

Your Fuse 4 should be okay otherwise you wouldn't get parking lights working on the front. Same for the headlamp switch, you wouldn't get fronts to work if the switch was bad.

 

thank you my brother it was a dirty connection