550 right rear flasher not flashing...

It must be a wiring issue. I don't know how you'd isolate it. There is only one (hard to get to) plug between the main RH turn splice (pigtail) under the dash and the tail light.

It's the blue wire if you can see the wire colours.

Check the resistance between the light cluster and the flasher unit (pin 5) if it happens again.

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Compare it with the resistance between on the left rear lamp (blue/black wire) and the flasher unit (pin 6). You'll probably need a long piece of wire to help with the resistance check.

A cheap wire checker might help if you don't make any progress.




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Unfortunately you've eliminated the easy solutions... lamp and lampholder.

When the flasher fails, are there any other (non-turn) lamps behaving strangely? All the rear lights use the same earth/ground. Finding the earth, however, would be the hard part. The location is not shown in the wiring diagrams.

 

Looking at the wiring diagrams, all the turn indication wiring for the right hand side comes from a single pin on the relay. i.e. front right, dash turn arrow and rear right. It should also power the flashing lamp in the hazard switch on the console when you turn on the Hazards. i.e. all the wiring is spliced together. It can't be the relay or wiring on the socket for the relay, otherwise the whole right side would not be working.

Having said that, sometimes Ferrari diagram "splices" are actually multiple wires crimped into one relay socket contact. If you see 4 or 5 blue wires squeezed into one socket contact, then I suppose the wiring on the relay socket could be faulty. Just seems unlikely, though (4 or 5 wires into one socket)

 

A third possibility...

The large splice in the OEM diagram with 4~5 wires coming from it, may be broken up into lots of 3 wire splices like this:

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With the Scud-ing relay installed, what happens when you use the Hazard switch? Does the Hazard switch flash? Does the turn arrow on the dash operate? If the wiring is as shown, it may help isolate the issue.

 

And the hazard pushbutton itself? Does it flash?

The output of the flasher relay also goes to the button.

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Actually, I've drawn that theoretical diagram incorrectly. There are two inline connectors going aft of the dash wiring. Maybe it looks something like this:

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If the button doesn't flash (when the hazard button is pushed), then either the splices under the dash are broken or perhaps just the button bulb has blown.
Continuity checks with an ohmmeter may help locate the break in the wiring

 

Pity. That doesn't help us narrow down the wiring issue much. The defect could be anywhere from the dash to the light cluster. On the V12's, that generally means pulling the car to pieces. Can you see any splices in the blue wire near the relay socket?

 

This specific piece gave me a tummy ache….
The defect could be anywhere from the dash to the light cluster. On the V12's, that generally means pulling the car to pieces

 

How vulnerable is the wiring behind the light clusters? If you put a lot of luggage in the trunk, does it squash the wiring?

How hard is it to access this area (to get to the inline plugs)

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If it's easy, then you can divide the wiring into two: Dash or rest of car. i.e. to carry out continuity checks.