2005 f150 4by4 brakes

The high mount brake light and the lower ones are on different fuse circuits. 1st off you need a wiring diagram booklet.

Year/make/model didn't post. The high mount brake light is not fused seperately. It should be on the same fuse as the brake lights. If it is not working check for a bad bulb. If the assembly is LED, check for power and ground at connector to light. If power and ground ok, suspect defective LED assembly

My info says hazard/flasher fuse in center console fuse box. You could also check proper colors at brake switch, the brake switch should be activated by brake pedal arm. Orange wire is hot all the time. White wire goes hot when brake switch is activated. Are the high-mount and lower brake lites both out?

The third or High Mounted Brake Light runs straight off the brake light switch on the brake pedal arm. You say you tested the "brake unit"? I assume you mean the third brake light assembly, and if you get power there when someone steps on the brake pedal, then the bulbs are bad.

The power flow for the brake lights goes like this: Fuse ->Brake switch -> Turn signal switch -> Rear lights. The only difference is, if you have a 3rd brake light in your rear window, that gets its power directly from the brake switch. It does not go thru the signal switch first. So, if you are having a brake light problem, and your fuses are good, check the 3rd light in the rear window, if it lights up (assuming the bulbs are good, no one ever changes them), then the problem is most likely in the turn signal switch, hope this helps!

If bulbs are definetly ok you can look behind trim on the left side in the boot, which covers the space between the boot floor and the window, you are looking for a green/red wire in a connector. this is the power cable which supplies both rear side mounted brake lights. You should check for voltage at this wire when pedal depressed. If no voltage you would need to trace this back to where the wire is joined in the loom as it feeds off the wire which supplies the high mounted brake light and you have stated that this works. Would still double check bulbs first though if you did'nt replace them. Hope this helps.

Did you check the fuse,#23 in JB center,this is hard wired to brake swicth, PK/BL should have 12v,if ok procede, wh/br will have 12v when brake is pushed on.if not replace the switch, jump the 2 wires and light sould come on, same feed goes to high mount lamps, does the info console warn of no lights, if you need more info, just ask. GOOD LUCK, RDJRP

clean bulbs socket out using the electronic spray cleaner for electrical connectors.

make sure u have put the right bulbs in. rear brake lamps are dual filament....

I don't know the model year, to look up wiring diagram? Do you have a high mount brake lite? Does it work? The highmount and lower brake lites are routed differently, the lower brake lites are usually routed through the multifunction switch, the highmount brake lite isn't wired that way. Did you check voltage at brake switch for brake lites, that is a fused circuit.
Usually the lower brake lites and turn use the same bulb filament, but, not at the same time.