We recently had a VW Passat B6 in with an indicator issue.
The dash was showing the indicator blinking at double speed.
As the rear lights are LED this was actually the control unit making the fast flash on the dash. The front and side repeaters were flashing at normal speed but the rear indicator on the left side was inoperative.
To rule out a possible ECU malfunction we plugged the car in and saw the codes:
1501 Rear Turn Signal Light - M6
984 Bulb for Left Taillamp - M4
As the rear lights have no bulbs this indicated a fault with the assembly.
To prove a fault with the rear light assembly as we noted the connectors are the same on both rear tail lights we removed both rear tail lamps and switched them around to prove the fitting was faulty.
The taillights are removed by removing the luggage hook with a T25 Torx screw. The trim can then be pulled back to give better access to remove the 10mm nuts holding the taillight in position.