First step would be to figure out where the connectors for the harness are, unplug, clean up, inspect wires near the connectors for corrosion or breakage, and reconnect. Second step would be to actually get down and dirty with a voltmeter and see what's happening with the signals.
Since it's affecting more than one signal, I'd suspect a poor power or ground connection (ground being more likely since the meters actually do deflect.)
Luck;
jky
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