Look under the hood near the steering column, inboard side. There is a rubber cap (grommet) with a wires coming out of it and a couple unused nipples. That's where. In my S70, I have to move a couple things out of the way to gain access.

And here's how I got my two wires through.
1) I make a probe out of a long, thin finish nail. (I've also used bamboo barbecue skewers and sharpened copper wire.)
2) Next, I take a long length of wire and fold it in the middle into a loop. This is what will pass through the firewall.
3) I attach the loop to the probe with electrical tape. Splicing with a paper clip straightened on one end works very well. I posted a diagram at http://david.maurand.com/firewall. (Hook the loop and crimp down to make it compact, strap the straightened end to the probe.)
4) If you can, cut the tip off one of the nipples. I just poke a new hole with the probe. Note: it's easier to grab the wire inside the car if you go through the bottom half of the grommet on the inboard side.
5) Using 10" needlenose pliers with bent-hook jaws, I poke the probe through the grommet about 2-3". Then inside the car, I locate my probe near the brake pedal and finish pulling through. Cut the loop off the wire and you have two ends.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)