[Laird Scooby]

In the 70s there were various aftermarket fuel computers marketed and if memory serves correctly, one of them used a device that fitted onto the speedo cable, hence three terminals. One was the +ve feed, second was earth (ground for our USA cousin) and the third was the signal for the "black box" that did the work.

I believe there were also aftermarket cruise control kits that relied on similar devices attached to the speedo cable and also trip computers that used them.

As for photos, i use postimages.org and the "Hotlink for forums" link, easily copied by clicking the blue button at the end of the link then just paste it where you want the photo to appear.