I've just become the charity recipient of an 84 240GL wagon.
I love it.
It’s in near perfect condition, 177k miles, always garaged California car, never been hit or painted, all accessories work, and I have all the service records back to 86.
Currently, it barely runs, and the mechanic’s report to the previous owner says:
“The engine is running rich. Checked the Mass Air Flow Sensor and found fault in input signal. The engine wiring harness needs replacement first.”

I talked to the mechanic, and he said the components could not be checked until the wiring harness is replaced, and due to bare wires, it’s a safety hazard.

I put shrink tubing and electrical tape on the exposed wires I could find (at white plug/connector near firewall and alternator power & ground wires). None of these wires or connectors appeared brittle or corroded.
I hate to put $1000. into a wire harness right away if that’s not going to solve the issue, (unless it truly is a safety issue). I would rather fix the specific problem and drive the car awhile first.
So my questions are:

1. Can I test the Air Mass Meter and ECU by bypassing the wire harness?
2. The car has had the AMM replaced 3 times, most recently at 163k miles. Does that sound right?
3. Is it just plain due for a wire harness? If so how difficult for a DIY job?

Thanks very much for any insight,
Jjs, San Diego