Possibly Enemy Intel 6 is the hardest piece of one to find in the entire game.
It does not help the situation that you have to drive over it during the snowmobile chase part of the level either.
To give you an indication where this tricky-to-spot Intel is located, it's about 1800 meters away from your goal, so it's relatively early during the course.
To stand the best chance of getting the intel, slow down a lot when you are about 1900 meters away from the final destination and you'll soom arrive at a small fence placed horizontally, there is also a hut on the hill to the right.
Pass the fence and just barely to the left side, this intel is lying a few meters across if you just go straight ahead.
The video below should help to illustrate the above text.
All 45 Enemy Intels are covered with both text description and video in our Call of Duty: Modern Wafare 2 Guide which is here:Call of Duty Modern Warfare 2 GuideEnemy Intel Location Guide
Enemy Intel #6 (Cliffhanger)