This cross-section of my Lego Wheatley model allows you to see how his eye was lit up in the original photo. A chamber of 2x2 trans-blue round bricks extends across the interior, allowing light to pass from a small window at the rear, through the model, and exit from the eye at the front. I used a high-powered LED flashlight to achieve the effect.
I've made up three Lego Digital Designer files for Wheatley. One contains the basic model, another includes the harness I used in the original photo, and the third offers an exploded view that'll help you understand how he is assembled. Click here to download the files, and remember that you'll need Lego Digital Designer to open the file. (Also note that these files were created with Lego Digital Designer's Extended mode, and therefore cannot be ordered from Lego.com.)
One last thing— I've posted a Portal 2-inspired tee design over on Qwertee, and would be very grateful for your votes! Please click on through to vote for I'm a Potato.