89 Cavalier Speed Sensor Problem

I have a 1989 Chevy Cavalier. It is the 4 cylinder 2.0L engine 5 speed with 175,000 miles. My check engine light keeps going off so I had the codes ran and it came back vehicle speed sensor. I had a new speed sensor put in when my transmission was being swapped with a transmission from a 89 grand prix ( identical transmission). I was just wondering what could be causing the code to be going off still. I have tried methods of resetting it. It sometimes comes on right away after starting and sometimes it varies a bit. After market cruise control has been added to the car and has not worked since the engine light has been coming on. Speedometer works but is off, especially while driving at highway speeds. Also my SHIFT light has not been working since this problem has occurred, was just wondering how easy this is to fix. And where this speed sensor is located, thanks!