I did this on my '89... I'm guessing the wiring is very close.
You can do this fairly simply by just adding 1 diode and with out any cutting of wires. BUT if you do it this way, the Foglight switch will light up when you step on the Brakes. The better way to do it is to add two diodes. the One would run from the Blue/red wire from the brake switch to the white wire of the Foglights. The second diode would be between the white foglight wire and the fog light switch so you have to cut this wire. this diode keeps the voltage from getting into the foglight switch when you step on the brakes.

I did this under the dash , right next to the Bulb out sensor.

Can you read electric schematics? I have a sketch if you are interested.
I used a dual diode that we had here where I work. You can buy what you need at a local electronics store ( Radio Shack)
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.no.net/ebrox/Tony's%20cars.htm