its a rather grueling process.
the metal bracing behind the dash is sharp, rusty, and painful.
post a WTB in the for sale/wanted section for an upper cable in good condition. i did it before like that and got an extremely low mileage one for a good price. a verrrry good price. ask around. we have a lot of extra shit laying around. we all drive the same car. as far as doing it, it unclips from it's little "seat" behind the gauge cluster. just snaps out of there. from there, you must go under the car and there's a short cable it joins to @ the transmission. remove it at the short cable, leaving the short one in the transmission. then, go up to the engine bay and remove the grommet holding it in the firewall.
NOW!!!! most important part.
its a bitch to try to aim it through the under-dash mess to feed the new one into the engine bay. so..... before you remove the old one, tie a good length of string to the end of it. as you pull it through the dash, it'll leave some string behind, and out the hole. untie the string from the old, and tie to the new. go into your engine bay and pull the string. the cable should come through peacefully.
re-attach all and be careful lining up the cluster.