I agree with el-duderino, if you are not confortable with what has been discussed in several other threads on that same subject then you should let a professional do the installation. If you do feel ok though doing the install here are some info which will help:
First check your stock amp and see what type of connector it has. The stock amp should be located in the cargo area on the left side behind the flashing. If you have CD changer the stock amp is underneath the CD changer. To check the pins you must eject the connector from the stock amp. This is the large connector that has several wires connected to it. The 3 types of connector I know about installed on X5 are either 15 pins, 12 pins or 42 pins. There may be other connectors but these are the only 3 I have seen so far on an X5. The 42 pins is actually built up of 2 separate connectors.
If you have either the 15 pins or 12 pins connector this thread should help you about the wiring and where to tap into for the sub signal. http://www.xoutpost.com/stereo-system...ew-member.html

If you have the 42 pins this other thread should help you with the wiring
http://www.xoutpost.com/stereo-system...info-here.html
Good luck