Most manufactured rear wire lowers come with the appropriate "holes" that you require to thread your wiring. If your lacking the appropriate "holes" creating them is very easy. All you will require is a dremel.
Please have a look at the last photo of this product listing.
http://www.gp-web.com/en/productspop.php?pid=2591
The hole you are currently attempting to thread the wiring through is a hole that is required for a screw that threads into the spring guide inside the mechbox. Using a dremel and the picture that I linked you as a reference you will require to cut out a small portion of the back of your lower receiver and thread the wiring along the outside of that smaller tube that you slide the stock tube onto.
Here is a link to an older thread that may be of some use to you.
http://www.airsoftcanada.com/showthread.php?t=100261
And another link to another forum that may help you as well.
http://www.airsoftforum.com/board/Re...ode=linearplus