Although such a diagram may exist it is very unsual for tow bar installers
(even Fiat Dealers) to use the manufacturers connections.
The current connection methods use a 4 or 6 way bulb failure, votlage
sensing unit that takes a single +12V feed from the battery. It then uses
sense wires to the rear light clusters (only take few milliamps) to switch
relays to power the trailer/caravan lights from the main +12V feed direct
fror the battery.
The voltage sensing loop measures the +12V voltage to detect that the
battery is being changed and the engine is running to switch a spur 12V feed
to the fridge circuit.
Quite elegant really. The oonly down side it that most installers use
Scotch-Lock connectors to spur into the existing wiring often leaviing no
room/loom length for repairs. Instead of tapping in many inches back down
the loom they often tap in 1 inch (2cm) directly behind the light cluster
connectors so when wiring problems occur one has no slack to manoever around
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