Emissive factor missing?
I downloaded this car model:  https://sketchfab.com/models/885d9f60b3a9429bb4077cfac5653cf9

After import/export trip through Blender, the emissive channel is not active.  Turns out that the exporter wrote an `emissiveTexture` but not an `emissiveFactor`.  The default `emissiveFactor` is `[0, 0, 0]` so as not to blast all models to full white.  But without the `emissiveFactor` being set, the `emissiveTexture` is multiplied by zero and has no effect.

(Also looks like blending may have been lost from the car windows, that's separate...)
diff --git a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py
index 4a4d6c50..3fa82525 100644
--- a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py
+++ b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py
@@ -109,8 +109,13 @@ def __gather_emissive_factor(blender_material, export_settings):
     emissive_socket = gltf2_blender_get.get_socket_or_texture_slot(blender_material, "Emissive")
     if emissive_socket is None:
         emissive_socket = gltf2_blender_get.get_socket_or_texture_slot_old(blender_material, "EmissiveFactor")
-    if isinstance(emissive_socket, bpy.types.NodeSocket) and not emissive_socket.is_linked:
-        return list(emissive_socket.default_value)[0:3]
+    if isinstance(emissive_socket, bpy.types.NodeSocket):
+        if emissive_socket.is_linked:
+            # In glTF, the default emissiveFactor is all zeros, so if an emission texture is connected,
+            # we have to manually set it to all ones.
+            return [1.0, 1.0, 1.0]
+        else:
+            return list(emissive_socket.default_value)[0:3]
     return None
 
 
['addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py']