I ordered the original part for T420 from ebayhttp://www.ebay.com/itm/Genuine-Lenovo-Thinkpad-T420-T420I-T420S-T420SI-Camera-Webcam-Kit-60Y9994-/180863823183?pt=LH_DefaultDomain_0&hash=item2a1c52ed4f
1. remove lcd bezel, using this tips http://forum.thinkpads.com/viewtopic.php?f=45&t=105165&p=681086#p681086
2. replace the small pcb under thinklight, the original one only have microphone without camera http://imageshack.us/photo/my-images/59/originalpcbwithoutcamer.jpg/
lenovo service center bulls**t, they said it is not possible to put camera there. I checked the bios, there is a webcam option there, and even after opening the LCD bezel, the cable and everything is ready.
NOW MY T420 HAVE A WEBCAM!!!