Either you made a mistake while replacing the harness, or there is actually something wrong. There is no reason for the MIL to light other than a fault in one of the monitored circuits.

Since the MIL showed up after driving 8 miles would seem to indicate that it is not a hard fault, but rather an intermittent problem, or a problem in a system that is only tested by the PCM only under certain circumstances. If you disconnect the battery, you'll erase the code, and it may not return until the PCM once again tests that system.

I'd suggest finding out what code or codes have set before erasing them. That way, you'll know if the next MIL is the same system, or something different.

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1989 300 SEL that mostly works, but needs TLC