98 3.8 tach problems

I have a weird problem with the stock tach in my 98 3.8 auto convertible and I need info as to what the causes are and how to fix them. I bought the car a year ago and the tach has never worked right. It registers only sporadically and now the needle is on the wrong side of the post. From the beginning it might show 1500 rpm after startup and it might show 100 rpm then drop to zero until I jump on the accelerator then it registers (whatever rpms) as long as the car is gaining speed but when it gets close to coasting speed the rpms drop to zero OR, like now, flies to the back side of the post and only rarely comes back to the right side and shows zero. Rpms have never been right. Now, first off, let me say I don't want aftermarket gauges and just need to know if I need to replace the ECM, PCM or just the gauge cluster. Since this is NOT a mechanical cluster, i.e. no speedo cable just electrical connections, how do the speedo, odometer and tach get their signals, what wires are they and where are they located?

Thanks from an old geezer that really likes his Mustang, STOCK!