installing a tach?

my 87 AC jag has no tach but has a place in the dash for it, have any of you added a tach to your sleds, if i can find a orignial tach, "good possibility on a sled this old" how hard is it to wire, looking at my wiring harness i see a phased connector that goes to nothing im wondering if this could be where the factory tach would plug if it had one? , anybody else done this ? or have any experince with this?

if its not a CAT, then it must be a dog!