What type of connector do you have? Did you compare the connector to the one showed in the article below? http://www.xoutpost.com/articles/x5/s...ctory-amp.html
If you have the same connector then 7 and 14 are one of the 2 channels for the stock sub. Your connections to 7 and 14 may be loose. One easy way to know if your connections are good is to try to connect the 2 wires you tapped into 7 and 14 to your subwoofer speaker and see if it moves the speaker driver.
What type of sub amp are you using? Does it has both high level and low level inputs? If it has high level inputs you do not need a converter. If it only has low level inputs then you will need to convert the signal out of 7 and 14 to low level. If you connected directly the 7 and 14 to feed your amp thru its low level inputs you may have blown it which would explain why do not get any sound.