I did the same thing, and here's a link to Digitrax with instructions. You have to move one of the wires on the plug in. You can read about here:
http://www.digitrax.com/tsd/search/?q=proto%202000%20e6%20mars%20light
It works great. Click on the first article on the page, it gives full instructions, and a diagram of what you need to resolder.
Mike.
My You Tube
I am trying to put a decoder in a Proto 2000 E6 DCC ready locomotive. I am using a Digitrax DH126 decoder that supports the Mars light that is on this loco. It works fine in DC but when I intall the decoder and program it, I cannot get the upper headlight, the Mars light, to come on. Is there a secret to programming this decoder on this loco to make the Mars light work? I use Decoder Pro for programming.