Faulty LED

[Duncan Grimmond]

Earthing to body/chassis is almost invariably the cause of the often seen "miss flash" through brake or tail lights to the "miss brake" light through the indicator commonly seen on cars over 8 or 10 years old. Body to chassis electrical continuity is fine on a new vehicle until perhaps the guarantee runs out after which the manufacturer has no care or responsibility.
An added earth on a new or 80 year-old vehicle is a perfect(ish) example of fit and forget.