It can be the Keyboard assembly, or the flex cable between the keyboard and the mother board.
1. You can check the flex cable where is bending 180 degrees and see if there is any sign of breaking.
2.check if you insert the flex cable in the correct way in to the keyboard assembly.
3.If you want to be 100% sure that the computer is working without the keyboard you can strap this 2 pins (picture attached) and you laptop it should be starting.
http://cyrozap.com/2010/08/18/macbook-core-duo-logic-board-keyboard-connector-pinout/