550 Brake lights still not working

If everything else checks out as you've mentioned, you should check the brake light switch, #9 in the attached. Located at the brake pedal.
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Oops, you are correct. Don't know how I missed that when I read your post, its the second line.

 

If I can extrapolate from a long and sordid history with Alfa electrics, it's never the running electrics like alternator, coil, Magnetti Marelli stuff, but with all the other electrical gear....switches, connectors, lights, etc.

I learned over the years that the first thing to check is the grounding of any piece of the system to the bodywork. I would guess that your rear lights are not making ground contact through whatever lug or screw to the bodywork is supposed to provide the return path. Find it, take it loose, clean the hell out of it with 0000 steel wool and reconnect it. I'll give you 50/50 odds on bad ground. Just a guess, but one founded on classic Italian electrical systems....

Good Luck.

 

It could still be the brake light switch. Some brake switches are dual switches for added safety to the cruise control system. These switches do fail on occasion which can be a hazard. If dual, it could be that one half powers the third brake light and the other half the normal brake lights. This also leaves open the possibility of a blown fuse as dual brake switches often are separately fed over two different fuses for the same reason. You checked for dampness, did you also check the fuses??
Last, the wiring harness to the third brake light is probably separate from the harness to the trunk, so it could be you have an oxidized connector somewhere.

 

There are 2 harnesses running to the back of the car and it looks like the 3rd brake light and the rear lights are in separate harnesses. I don't see any grounding of the lights at the lights, at least not on the schematic, there is most likely some common ground points to ground many things. Not sure if you have the WSM, so just in case here is a excerpt that may help you trace. They will make you dizzy staring at them, but they helped me sort out some electrical problems. I highlighted the rear stop lights. Good luck.
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550 wiring diagrams are some of the worst and most inaccurate ever produced by Ferrari. I typically ignore them because the errors almost always costs me more time than just figuring it out blind.

 

I know it sounds dumb but it might be faster to wire the lights fresh. I think you are lucky you have lost connection to such a simple item.

 

To me that would be a repair of last resort. Sorry but it will more resemble a farm tractor repair.

You have power going from the switch to the back half of the car. The diagnosis at this stage will not be difficult and a quality repair should be done.


Have you tested all the bulbs? I have seen many cars with every brake light bulb burned out.
Have you tested for ground at every socket? If not why not?
Have you tested for power at every socket? If not why not?


Following the factory diagrams is not difficult. Trying to reconcile why circuits cannot electrically function as presented is difficult.

 

+1 Get out your multi-meter and check for power when the brake is applied. And check the bulbs. If you have tail light operation, then the ground is likely fine.