Did the same thing with my 89 740 having the fogs go on with the brakes. It was easier then as both the left and rear fogs were present so I didn't have to dremel an opening for a new holder. I put my diodes in a "Y" up at the front between the brake pedal switch and fog light switch. The fogs would stay on when switched, but otherwise only went on with the brakes. I actually had someone tell me the lights were now too bright.

My only problem was I didn't use beefy enough diodes so they only lasted a couple of years before frying and I never got around to replacing them before selling the car. I'd made the "Y" using disconnecting crimp terminals so I could pull it out and make everything original lest it cause the next owner confusion if they ever looked up under the dash.

For a sedan I probably wouldn't bother again, but for the wimpy tail lighting in my wagons I'm starting to think it would be a worthwhile project what with the ever increasing number of impatient tailgaters on overcrowded highways. I don't have a problem tapping the brakes to annoy their retinas with bright brake lights if it wakes them up to the fact they're driving way too close for safety. It doesn't matter if a rear ender is their fault, what matters is my precious old Volvos getting written off with just a small bit of damage.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now