If you have the stock audio non premium chance are that you have a 12pin connector instead of the 15 pins which I was refering to in this post http://www.xoutpost.com/showthread.php?t=5912.
You should be able to locate the connector to the amplifier under the CD changer in the left side of the trunk behind the flashing. To look at the connector you will have to eject the amplifier connector. Pretty easy with the help of a screwdriver. You may need to remove the CD changer to have better access to the amp connector.
Once you have ejected the connector you can look at the pins. As far as I know for a non premium sound you should see 12 pins on the connector. Look at the number of pins and not the number of wires since all 12 pins may not necessarily be connected to wires. If you have the 12 pins connector then you can tap into wires #11 and 12 (instead of #7 and 14 described in my post) to feed the Sub amp assuming that your amp is equiped with high level inputs. If it has only low level inputs you will have to add a line-out convertor as explained by EKAZ. The pin # should be written in very small characters on the connector. To locate the pin# on the connector it helps knowing that the two thicker wires are +12 (usualy Red or Red and Yellow) on pin #1 and ground (usualy black or brown) on pin #3.