It was a combination wire leading from the battery. It is now an obsolete item that can be picked up from some of the Cougar Vendors. People typically have problems with their ground on the 1970 Cat when they replace it with a normal wire.
My wife's 70 Mach 1 is an all-original unrestored car. Mustangs and Cougars are assembly line brothers. That means that they are the same under the skin.
OK, now with that out of the way, I can tell you that a 70 does not have a separate braided ground strap at the back of the motor. The factory negative battery cable had an additional lug in the middle that fastened to one of the voltage regulator bolts, thus giving the chassis ground, and then on to the front of the engine block near the bottom (engine ground). You can get the correct cable at any Mustang vendor.
On my Cats I went with 0-gauge battery cables for extra grunt power to the starter. For the negative cable I used a short 0-gauge battery cable to the voltage regulator bolt, then a short 'eyelet to eyelet' 0-gauge cable from there to the block ground bolt. If you ground your cat right it will do away with the flickering headlight syndrome and also eliminate a bunch of those pesky sneak (feedback) circuts.
i am just getting started on sorting the electricals on this car. I noticed that all the light are very dim, it does not have the correct ground cable arrangement, i will fix that this weekend (thanks a bunch for the info y'all) Also the sequential box looks smaller than others i have seen, probably a solid state replcement unit or somthing??