Malfunctioning brake lights
I am having same trouble in my 2001 S-60. LED window brake strip works fine but both L/R brake lamps are out. I have swapped relays and shunts without any change in symptoms.
I have also had a very intermittent problem where rear parking lights stay latched ON after the car is shut off. Pulling out relay RMI5 and re-inserting it clears the malfunction.
I think these two errors are likely related, but not sure where. I will try the lamps, to eliminate gremlins, but the original ones were apparently OK before.
What does a shunt do? When I check with ohmmeter they read zero ohms across all terminals. The schematic shows there is a resistor network across certain terminals but to my meter the unit's terminals all appear joined as one, electrically. Tested 3 shunts, all with same properties.