Two pictures. One end connects to the point shown in your photograph, the other to a small hole in a lip that is between the two flanges of the hood hinge connection on the driver's side.
Unfortunately, as I have mentioned elsewhere, SplendidParts no longer ships to North America. Possibly Gil at OGTS can help source one. The spring is "at rest" when the headlamp doors are open, so you can measure the distance from the post to the hole to see how long the "at rest" spring needs to be, then add the distance when the doors are closed. It is not a heavy spring, and you might be able to find a suitable one at the local hardware store.
Hope this helps.
The spring is under tension when the headlamp doors are closed (headlamps off), when the connecting rod is extended out of its casing. My guess, and a guess only, is that the spring is intended to reduce vibration when the connecting rod is so extended. It is a fairly light spring and I doubt that it has any material effect on opening the doors. If it did have such an effect, then it would have the opposite effect on closing the doors, as the driver would be working against the spring tension to close them.
You missed what I was saying. The spring has nothing to do with holding the doors in place -- it is essentially at rest when the headlamps are on with the doors open. What I was suggesting was that, with the doors shut (headlamps off), the connecting rod is fully extended out of its casing and my thinking is the spring controls some of the vibration that the extended rod might experience.
Well, at the margin, it does help push the handle forward. When the doors are closed (lamps off), the spring is under tension. In pushing the handle forward to open the doors, the cable moves in the direction in which the spring is relaxed -- in effect, the spring is helping pull the end of the cable back into the casing.
As I said, it is not a big spring, so it would take a pretty fine instrument to measure the difference in forces between opening and closing the headlamp doors.
But in reality, the difference you feel is simply a matter of kinesiology. When pushing the handle forward, you are using both your arms and your back, the musculature in your back being quite significant. When closing the doors, your back plays little to no part -- its all arm and shoulder, with perhaps some leverage gained off a leg.
Ultimately, I was not as good at it as I wished to be, but I spent years on and off of a wrestling mat trying to understand balance, leverage, and, of course, strength. Moving anything heavy is easy, so long as you can get your back into it.
That light must have been added later in 1970 because my original '70 GT had the rear window defroster but I do not recall it having the lamp. My GT back then was an early model for the year, the car having served as the dealer's demonstrator until I purchased it in September of 1970. The 1973 FSM shows an indicator lamp coming off the relay. Interesting, but the 1970 schematic also show an indicator lamp