MTH GP7 Headlight Q

Perhaps the reason why you did not get a response after 11 hours is because this is posted in the wrong forum.  I moved it to the MTH forums which is where it belongs since this is not a topic about a 3-rail traditional toy train rather specifically is discussing an MTH product.

https://r.search.yahoo.com/_yl...H8qDAO0Xh68dgMTp19Y-

Above is the link to the parts pdf download for that GP-7.  Judge if there's enough lead length to swap the headlights,

one (#58) is  80 mm the other (#19) is 85 mm

Pdf also attached

ok, finally got a chance to look into this and fixed the problem. ill mention it here in case it happens to someone else. thanks for the advise above although I didnt notice it until yesterday. when you remove the body of the locomotive there are 4 springs on each end that make contact with electrical pads on each end of the locomotive. I noticed one of the springs on the offending end of the locomotive was noticeably shorter than the other 3. they are just pressed into a housing, so I pulled it out some and put it back together. presto, head light works. 

One of the most common issues with shell lighting on MTH diesels is the spring contacts.  It's also typically very easy to fix as you have discovered.