my front left turn signal wont work, the signal on my dash board doesnt work, and the signal in the rear blinks rapidly. but the whole right side is fine. this is aggravating, I have changed the lightbulb in the front and nothing happened. it still doesnt work. any ideas?
thank you
Hannah
If you replaced the bulb then you have a problem with the bulb socket and/or wiring to the front left turn signal. You could start by cleaning the bulb socket. After that its getting to work with an electrical multimeter.
Like cigroller said, the problem is more involved than just the bulb. But if your left front turn signal AND the left arrow in the instrument cluster do not light up at all, but the left rear and right sides work fine, you almost certainly have a failing Multi-Function Switch (turn signal, wipers, high beam all in one) on the steering column. Kind of a common problem on Fords.
Try the hazard lights. Do all 4 corners come on then? I had a 97 Taurus with a multifunction switch that “failed” on me - the turn signals worked fine, but the hazard lights didn’t light up all 4 corners. I didn’t know until the first time I had to use them, when the car was 13 years old and I had to park temporarily in a hospital’s loading zone during an emergency run in (no employees around after hours). I removed the MFS and opened it up - it was clearly just a contact corrosion issue and was EASILY fixed with a little sandpaper… turns out copper switches corrode a bit after 13 years without use. Who woulda thought? 
You don’t say what year your F-150 is, but I had the same problem with my '98 F-150. I bought the truck for a low price partially due to steering column damage from attempted theft. I replaced the steering column with one from a salvage yard which, naturally, came with another multifunction switch. The problem disappeared with the installation of the new (used) steering column and its included multifunction switch. If you want to replace or remove your multifunction switch for inspection, let us know. They are easy to replace, but there are some little tricks that you will need to know to do it with minimal headaches.
I’ve had 2 multifunction switches go out it has some of the symptoms you listed. The right side was fine hazards worked, my left side would work but would either freeze on the dash or flash real fast but in both switches I actually had smoke come up out of the hazard switch area and the turn signal knob area. The first time I called dealership and they said it was multifunction switch & it was recalled so they replaced no charge 2nd time was recently & I replaced myself. Hope it helps you