Hey i can help. The guide i followed is here:
Ferells Tutorial . The first time thru, i followed the tutorial and it didnt work, so i got a new switch and it still didnt work... so then i thought my soldering job was bad, but it turns out that wasnt it either. The problem all along was that i, along with you, didnt follow the directions correctly

. At the URL i posted above, there is a link to Xboxmod08's tutorial.... there it explains how not only do u have to modify the memory card, but also do a bit of soldering of the yellow wire to a point on the xbox mainboard

hope all goes well with ur modding
Gerzand