1998 Ford Escort help needed

Hi, this is chris and I have a 19984-door ford escort se. The car has 144,000 miles on it and has been relatively maintenance free up to this point. I have pretty much just had to put on a new master cylinder in the 3rd year I owned it and a new radiator in 2005. The radiator was leaking around a plastic ring but instead of just being able to get a new plastic ring and put it on and re-seal it they make you buy a whole new radiator but anyway to the problem(s) at hand. First,I noticed today that my speedomter stopped working when i went home for lunch from work. Any ideas on what this could be or how it can be fixed? Secondly, The negative ground wire harness I had was starting to get corroded pretty bad and the car was not starting up as usual. I had to give it some gas whenever i started it to get it to go. After it was running it would run fine and would idle fine in traffic at lights and so on.(I called the dealership and they said they didn't sell just the black plastic part and that i would have to buy the whole thing battery cables/wires and all for $397.00). I went to pull-a-part and got the black plastic part I needed off a 1997 ford station wagon and had it put on this morning and it seems to be working better (so far today).When I went home for lunch the service engine light came on and from driving home and back to work it didn't go off. Any ideas on this? Could disconnecting the battery and putting the new harness on have anything to do with it or throw the computer off any from where it was previously set? Thanks for any help that can be provided.

The cable may have broken on the speedometer. Disconnecting the ground cable should have caused the computer to reset automatically. If you take the negative battery cable off for 15 minutes it may make the computer reset for you which would make the check engine light go out. Also, if 1 of the ground wires on the harness isn't grounding good to the negative cable it would make the CEL go on. Grab the harness and pull and shake on it. It may not be making good contact. Are your cables on the battery clean and tight?