1988 Continental Digital Speed ODM Problem

I have a 1988 Lincoln Continental Signature Series V6 3.8L, About a year ago the speed speed odometer quit working on it. Then after that, the transmission went out. After I replaced the transmission, the speed odometer started working perfectly again. Then about a month ago the speed odometer only reaches to 4MPH. I'll be going 30/40/65 MPH and it will still show 4 MPH. So I'm guessing it's not broke, and I don't know what the problem is. Where does the speed odometer connect to. I took it to the transmission shop, then fixed it when trans went bad. Now it's stuck on 4 and I dunno how to do it. The transmission shop closed down too :/. Thanks for any help guys :)