I'm no auto electronics expert, but I have added LED strip lights to the rear reflectors on 3 cars I've owned, including my '21 Forester (see pic below). I would take an educated guess that the USA wiring harnesses used in your car are entirely different than wiring harnesses Subaru uses in other countries outside of the USA where rear fogs are legal. If that is correct, there is almost certainly no hidden front fog light wiring in the rear cargo area you can access. In my case I connected a 4-pin trailer wiring harness to the unused factory-installed trailer wiring plug that Subaru places behind the rear cargo trim for the towing hitch accessory add-on. I simply connected the new light's wiring into that 4-pin harness to illuminate the rear reflectors. My rear reflectors are illuminated now whenever the car's headlights are ON, regardless of whether or not the front fog lights are ON or OFF, which I am perfectly happy with. I suspect the only way to do what you want in a car for the American market is to somehow run rear wiring all the way to the front fog light wiring as you already suspect.

 

Interesting question. Given that North American Foresters are produced in Japan, is it possible that the wiring harness might contain some traces of the same DNA related to features included in other markets? I'm going to say it's entirely possible that a lead from the front fog relay might extend towards the back.

The difficulty would be in identifying it. I recently added front fog lights to my 2019, so I've been thru the Factory Service Manual wiring diagrams to see what options I might have for some modifications I too was thinking of. I can tell you that the manual pages posted by SOA do not contain any mention of rear fogs, so using that documentation as a guide for finding a buried Easter Egg isn't going to yield gold.

I think you'll have to assume that a home run to the front will be required. You'll T off the front fogs, and likely add an interrupt switch so that you can have them off when not needed. That line feeds a new relay with appropriate fuses to provide fresh power the bulbs in the back. You can pick up power out back from the Trailer harness connector in the way back, so all you are running from the front is essentially a signal line to the relay.

 

I had the same thought before as some of the aftermarket tail lights have that built in fog lights but I believe the harness that goes to the tail lights does not have the extra wire. The tail lights that have that fog light feature is this one. OLM USDM CS Style LED Tail Lights (Smoked/Black Base/White Bar). So I would assume running the wire from the rear to the front would be the best bet, probably even run it to a separate switch if you don't want to tap into the fog light wiring.

 

I'm about as susceptible as anyone to cool gadgets - but please do not install rear fog lights unless you promise ALWAYS to have them off if there is not thick fog. I've driven an above-average amount in Europe for an American, and believe me, you don't want to be in heavy winter weekend Sunday evening traffic with hundreds of Joe Average French drivers, who all have rear fogs turned on on their cars. The net effect is, it's hard to notice anyone's brake lights - scary and dangerous. Another observation, from having observed plenty of Japanese cars in use in Europe, in many cases the required rear fog is just one lens of what in the N American market is a backup light, with the EU model car having only one backup light. I don't know specifically about Subarus, though.

 

Connect the positive wire of the rear fog-light to the parking lights, then connect the fog-light ground wire to the fog-light relay ground. The rear fog light will then switch with the front fog lights. An on/off switch can be added for more choices.

 

Thank you all for the great responses! This isn't a mod I'll likely do (or at least not in the near future), but I was curious from a hypothetical perspective. If I ever do add those lights I would probably leave them as extra brake lights and nothing more. But it's nice to know there's options!

 

@GreenRubasu99 If you can get hold of a wiring diagram, you might find there is a potential solution in the trailer wiring connector.

We in the UK only use rear foglights when visibility is less than 100 metres/yards - Be that rain, mist, fog, snow etc.

Orrrrr you could look into a foglight switch from the UK/Europe and a set of rear light housings from said country to have the rear foglight options and maintain the factory look inside and out

 

Just a reminder that the lower reflectors are reflectors in Europe too. For the 2018-model, one of the reversing lamps is being used as rear fog lamp.

@TMX Although the European trailer connector will include wiring for a rear fog light (as it is required also on trailers), I believe the North-American trailer connector will be different and not include that wiring.