unless you have a bad short, the 87 harness is pretty reliable.
I'll have a complete parts car in my lot SOON... and the harness will certainly be available.
I would hope the speedometer still works, as it's mechanical in the 124... I'd think the entire harness under the dash would need to be pulled... I'd go looking for the short that killed it before changing it though.
I'd also visually verify the harness was bad before changing it. the mechanic may be assuming it's bad because the 94+ harness is known to fail, and he's lumping odd problems into that diagnostic. the 87 harness does NOT suffer from the degrading insulation of the newer vehicles!

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!