Hi Yves
The best I can do is refer you to the parts diagram.
http://www.bachmanntrains.com/home-usa/dwg/dwgs/H833X-IS001.PDF
It appears the LED either plugs into the front or has wires leading to the inside. If you can remove the shell, you should be able to see how the LED is connected.
Hope this helps, others more familiar with the loco may comment.
Regards
The best I can do is refer you to the parts diagram.
http://www.bachmanntrains.com/home-usa/dwg/dwgs/H833X-IS001.PDF
It appears the LED either plugs into the front or has wires leading to the inside. If you can remove the shell, you should be able to see how the LED is connected.
Hope this helps, others more familiar with the loco may comment.
Regards