It's probably using a Wheatstone Bridge type circuit to monitor the loads from the rear bulbs, and light up a warning light if it is out of balance. If you blow one on either side, it will even the load.
In Germany, they do not have the light duplication trick like Japan and USA. But it is illegal to drive without a full set of spare bulbs. So I suppose you are stuck with replacing the blown one as soon as it happens
Joe
PS Know what you mean about subtle on the rear panel. A number of times I have come back to my car in bad weather when the rear is mud smeared to find someone has rubbed the centre panel to read what the car is.
__________________
Black Betty [Bam a Lam!] '93 UK spec, still languishing
Betty
Jersey Girl Silver '92 UK [Channel Isles] 40K
Jersey Girl @ Mersea
Candy Purple Honda Blackbird
Plum Dangerous
White X2 RVR Mitsubishi 1800GDI.
Vantastic
40,000 miles Jersey Girl