If you're talking about the OD wire for auto tranny where it goes out the front of the shift lever box - - -

Cut it off down in the tranny tunnel so you have a short bit to pull up from the top side.
Then, from the top, with the shifter housing cover pushed over out of the way, pull the old wrecked wire up and out.
Now use needle nose pliers to thread new wire out thru the grommet. That takes a lot of patience.
Now go back down under and splice the new wire to whatever you've got underneath - either existing old good wire or maybe you ran new wiring around the engine-tranny area.

The grommet is the tough part. First time, I removed the grommet so I could thread new wire more easily. That lets lots of noise and maybe rainwater into the car. I had to use plumber's putty to seal up the hole, not so good.

Next time, I left the grommet in, and used needle nose pliers and LOTS of patience to thread new wire out through the grommet. I don't think I could ever get one of those grommets back in without removing the shifter box.
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Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).