This is a great mod. I just completed it on my wife's Taurus and it works great - loud audio, great quality, no fuss! The car has the CD player but not the tape player. I wanted to share a few things I did differently:
1.) Instead of removing the wires from the harness, I cut the wires and spliced into them using crimp butt connectors (original wire + new wire twisted together in one end, other side of the original wire in the other end). These provide very reliable connections as long as your crimp tool is any good (I used an Irwin 8-inch crimp tool, model #2078309). Note: if you want to cut the wires a decent distance from the harness, you will find that they are located inside a metal foil. This foil is grounded and prevents interference on the audio lines that would otherwise come from the power lines located right next to the audio lines. To keep the interference shield in place, after making my connections I wrapped everything back up in aluminum foil. If you don't put the shield back in place, you will hear a whine noise that changes pitch depending on how fast your car is revving.
2.) Instead of an audio jack connector, I just cut into an audio extension cable. This allowed me to have the wires I needed without having to solder anything to the connector. It also is great because the audio extension cable is shielded so it can help prevent interference.
3.) Since the original wires were left in place, I can still use my CD player. If I plug in my MP3 player, it overpowers the CD so that I hear the MP3 player instead. However, just to make sure that I'm not interfering with the MP3 player's signal, I created a 'silent audio CD' by burning 60 tracks that are 1 minute long and have no noise (see http://duramecho.com/Misc/SilentCd/). Whenever I want to listen to my MP3 player, I put the silent CD into the CD player. This technique is usually used when people add AUX input to cars with CD changers because they can keep the silent CD in one of the changer's slots and easily switch to that CD.
4.) Finally, I purchased a 12v to USB device (http://www.amazon.com/gp/product/B0042B9U8Q/ref=oh_details_o06_s00_i00?ie=UTF8&psc=1) and an iPod audio+sync cable (http://www.amazon.com/gp/product/B003L13BNI/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1) so that I can listen to my iPod and have it charge at the same time.
Thanks again for the write-up. Very detailed and helpful!