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On a 2003 Saab 9-5 Aero, only the left rear fog light lights up. The right side has a bulb but when I switch the bulbs, the left still is the only side that lights up.

I thought it might be a wiring issue, however a friend mentioned that his Porsche only has wiring and bulbs on the left side for the reverse fog light.

Is the left side really the only side that should light up for a pair of rear fog lights?

(The fact that the right side has a bulb in the Saab seems strange to me, but maybe it was easier to make them symmetric during manufacturing).

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Your friend's Porsche may be like that because, I believe, in Germany you only have 1 rear fog light so that it stands out against brake lights. –  Parker Jun 3 '11 at 12:07
    
@Parker, yep, I just posted an answer where I found this was also intentional with the Saab. –  jzd Jun 3 '11 at 12:14
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Both of my Audis have only the left rear fog active from the factory. This is a common request though and apparently they make a kit to install the second one. As others have mentioned, this is apparently to prevent them from being mistaken as brake lights. I believe on RHD cars it is the right one that is active. –  Sean Reifschneider Jun 4 '11 at 6:31
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2 Answers 2

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I did some research on some Saab forums. I found that typically on US bound Saab 9-5 a bulb is installed on both sides. However, the metal circuit on the passenger side is cut to disable that side. This is to prevent the rear fog lights from appearing as brake lights (there are arguments about the legality of dual rear fog lights in certain US locations).

It appears the circuit can be soldered back together and to enable dual rear fog lights.

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This seems to be a swedish thing - both of my last two Volvos (XC70 & XC90) have had exactly the same "feature". –  chris Jun 4 '11 at 14:21
    
As an additional datum to confuse the issue further: My Volvo 940 has rear fog lamps on both sides, but the lamps on the left are brighter. –  William Cline Jun 6 '11 at 21:26
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The legal requirement (in the UK) is for there to be at least one, however where your car is fitted with two rear fog lights they should both light up, otherwise you could fail an inspection if the police decided to stop you.

All the cars I have ever owned have had 2 rear fog lights, but you do see a lot which have one side fitted with a red fog light and the other with a white reversing light.

I would advise checking the wiring.

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I've seen a few cars that have lenses both sides (usually built into the cluster), but only bulbs/wiring on the driver's side. If bulbs are fitted both sides I'd expect both to work however. –  Nick C Jun 3 '11 at 9:23
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