check engine light '89 SEL

the check engine light keeps coming on in my '89 SEL with 168k miles. I took it to the MB dealer & they said the codes can not be read on my '89 ???!!!
I have the MB service manual & it talks about grounding pin 3 on the X11 block on the firewall & reading the pulses. The pin 3 on my X11 block has no wire running to it. I only have wires in 1, 6 &7. I replaced the O2 sensor & it didn't help. My wiring harness looks a bit beat. I read an article in where a guy replaced the harness on a '95 E, he was having similar problems. He said it was no big deal. I looked at mine & it looks like a big deal. When the light goes on the car runs rough & sometimes stalls. It's real random. I can drive for days without a problem and then take a short trip & have the light go on 6 times. any ideas, it's driving me crazy
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