Not too hard... The first job I had to do after Orange '74 arrived....
The levers that operate this cable and the engine cover release are in a housing set into the driver's side door frame, rear.
1) Decide what besides the cable itself might have to be replaced. You didn't mention your engine cover, so I assume that this housing and the levers are OK and it's just the cable that's bad. If the engine cover release looks bad as well, or if the housing is cracked, you'll want to replace those as well - parts available from Midwest Bayless and Vicks, both in the "Vendors" section of the wiki.
The cable goes through a black rubber-coated sheath that end in the rear trunk at the latch. Both ends of the sheath have a little metal grommet that fits into the mounting brackets at the latch and the lever. If these grommets aren't there, or if the sheath looks at all ratty or rusty, replace it too.
2) Remove the old cable. It's fastened to the end of the latch's operating arm in the trunk, and it may be easiest to just cut it. Now you can pull the cable out the same way you're opening the trunk now - when you tug, it'll just keep coming until it all out.
3) IMMEDIATELY block the rear trunk lip with something so that it cannot be closed. If it does close, you will be totally screwed - there's no way into the rear trunk that doesn't involve cutting sheet metal (There is supposed to be a backup release cable of heavy monofilament line routed to the engine compartment. Don't count on it, not even if it is there).
4) If you're going to replace the sheath, just start at one end or the other and work it free, unclipping it from the body and pulling it out. You already know how to remove the side air vent. The only tight place is where it passes through the front of the engine compartment under the inside air vent. Then replace with the new sheath, threading it through the same path as the old one, and setting one end in place at the rear trunk latch and other in the housing for the levers.
3) Once you have the sheath in place (either because the old one was OK or you installed a new one) you install the new cable. The cable has a little metal stop crimped onto one end - that's the lever end. Start at the lever, thread the plain end of the cable into the tunnel through the side of the lever, out through the little hole in the middle of the tunnerl, through the lever housing into the sheath, and all the way through until the end of the cable emerges at the latch. Grab it with pliers and pull until you have the ball solidly seated in the lever.
4) Now pass the end of the cable through the hole in the latch's operating arm. This is kinda cramped and it may be easiest to just unbolt the latch, pass the cable through, and then bolt it back up. Tie it off or use the little crimpable sleeve that Matt's cables come with it.
5) Test the thing by forcing the latch into the closed position and then seeing if the lever in the door opens it properly. Several times. Do NOT test it by closing the lid (see #3 above). When you're sure it's good... you are done.
