2006 Skoda Octavia rear light cluster. 

I have been adding an after-market trailer connection.  In establishing which wires did what (For use by the bypass relay)  I discovered that the "Side light" filaments glowed at reduced brightness. See the picture. There are 4 bulbs in the main cluster. Top and bottom bulbs are twin-filament for Brake light and Fog, plus, presumably, tail lights.  But there is no wire in the 6th plug position - in fact there is no pin in the plug in that position - and that is the position which connects to the 2nd filament of top and bottom bulbs.

It is evident that those two filaments cannot light at all - but they do at reduced brightness. (Around 5 Volts I believe).

How do they do that?

Connected wires are Brake, Indicator, Reverse, Earth and Fog.  Just 5 wires when there are 6 bulb terminals!

I picked up the tail light connection from the cable from one of the two separate small base-less bulbs mounted separate from the main cluster.  These light at full brightness.

Hi MileHolroyd, Yes I know of the Canbus problem. There are a number of suppliers who offer after-market cheaper towbars, and bespoke wiring, to avoid the Canbus problem; these do not need to have the car's processor re-programmed. A Bypass relay is used to supply the towing socket with 12 volts taken from the battery. High impedance connections are made to the car's lamp cable to "copy" the demands. And yes that means running a 12V power cable from front to back.  You may consider the Skoda method preferable if you can afford it!

 

I have been so puzzled by the glowing top and bottom bulbs that I have done some more tests.

I have been applying my multimeter and viewing the Brake light illuminated filament through dark glass.
It seems that the low wattage bulb element in both top and bottom bulbs is never used; a big relief for me because otherwise a law of nature would have to be rewritten!
The high wattage elements glow when side lights are demanded. One bulb is supplied with just under 2 V DC and 2.5V AC and the other bulb has only 1.9 V DC and 2 V AC. Both high wattage elements get the full 12 volts when brake light or fog light are demanded.
I still wonder why the lighting system has been designed like this.  I am assuming that it is the same for all Octavias and that I have not got a peculiar fault in mine.

Is this an estate or hatch? The clusters are very different in terms of bulbs.

Sent from my Nexus 4 using Tapatalk

As above, one of the many unknown oddities on the MkII Octavia.

 

On the hatchback the rear clusters are dual/twin filament (21W/4W) for the top and bottom functions - brake / tail and fog / tail.

 

On the estate they are only single filament (21W), the CANBUS system alters the voltage to reflect the required function, low voltage for the tail light function, increased voltage through the same filament for the brake light function.