Date registered: Jan 2011
Vehicle: 1985 190E 2.3-16
Location: Versailles, KY USA
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What do you mean by 'wiring harness is dead'? If it has no power at all, that's probably from a common problem: battery not grounded properly, + cable from battery to main wiring block has an issue, maybe just a corroded connection, might be a blown main fuse, could be a lot of things, most of them won't be fixed by replacing the entire wiring harness.
Sounds like this mechanic is playing 20 questions with M-B parts because he can't diagnose a problem. And there's a good chance that they'll mess something else up replacing the harness, that's a major ordeal to put a new one in. Heaven knows what labor will add up to on a job like that.
If it was mine, I'd break out the voltmeter and start tracking a dead wire back until I found out where power was present. Your real concern isn't another mechanical failure, it's that whomever is working on it hasn't found the problem.