Are you able to boot up using Target Disk Mode (TDM) ? Disconnect the battery following this guide [guide|3024], then press and hold the power button for 15 secs to fully drain the logic board. Now plug in the MagSafe charger and quickly press the ***T*** key to enter into TDM.
Reference: [https://support.apple.com/en-us/HT201255|Mac startup key combinations]
TDM bypasses POST and leverages Intels HD Graphics which is within the CPU bypassing the discreet NVIDIA GeForce GT 330M GPU logic. You should see a FireWire Icon on your screen do you?