[idmetro]

I haven't done this particular job either but have fished other cables through tight spaces and have a couple of tips to offer:
1) Using the old cable to pull the new cable into position is the best way, just make sure your attachement between the two is much stronger than you think you will need so they don't separate halfway through the job.
2) As stated you have to determine which way the cable will go in. Sometimes you will have to attach a line/cord/wire (picture hanging wire works great; its strong, flexible and cheap) to the end of the old cable pull the old cable out leaving the line in its place, then attach the line to the new cable and pull the new cable into position.
3) Once you have the two cables tied together (or the old cable and a line or the line and the new cable) use some tape (electrical, duct, whatever..) to cover your attachment points, make the overtape job as slim, smooth and tapered on both ends as you can make it so it will slide in/out of the tighest spots, You could even apply a bit of silicone lube to the overtape spot to help it slide through.
4) If while pulling things in/out you get things "stuck" pause, reach around in there wiggle things, give them a slight twist, pull back then try again just do not apply brute force or you will likely be graduating to the total disassembly mode to get your cable through whatever tight spot you had gotten stuck in. If you got frustrated and pulled too hard you probably pulled your fishwire apart from the new cable so now you have no guide to get the new cable into the right position.