Need Help Please

I am trading in my 93 s-10 blazer low rider to a guy for a 96 avenger and $4,000. It's a good deal and we're both happy with it but i will need help with the avenger.

OK here goes:

The avenger has some sort of electrical problem. Oh, first off it's a 2.0 5 speed. What it does is the speedometer jumps around and dies and the check engine light comes on. The car runs great but it seems to have a ghost in it somewhere. There are no trouble codes found and it the guy had it at a couple garages and they said it might need a new wiring harness. Well my question i guess is that i want to know if this is a common problem first off, is it hard to swap the wiring harness and does the same wiring harness control the speedometer and the check engine light? Everyone is at a kind of loss and since i live in Cali i'll need a smog check and it won't pass with the light on. Any help or coments would be greatly appreciated.

Thanks for reading.:screwy:

Well the deal went through, i did an obd scan on the car and the only thing that came up was the code that says vehicle speed sensor no input so i figure that should be an easy fix, i hope, anyway i'll post when i get it up and running. Oh i posted earlier saying that it had no codes, that was from the guy i bought it from but that isn't the case so thats kinda good. So far it seems like an ok car, looks good. The breaks will also need some attention because i can feel a pulse when the pedal is pressed but some rotors and pads and a good bleed job should fix it no problem. Thanks for looking and i'll post any problems or fixes i do so maybe i can help someone else.