you have "leakage current" when you use your turn signals that is feeding back to the guages is my guess,probably thru ground. if there is a voltage relay for instrument lights,the "hot" 12 v from the clock would weld the contacts either open or shut,but i don't think there is a relay. the leaking oil pressure guage is what i would suspect. oil that was wiped off the circuit board still has slime that conduct voltage.
take the instrument cluster out,take all gauges out and spray everything with "electronic contact cleaner" then wipe with coffee filter,it's lint free.

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72 250 CE-5spd-gone
79 280 CE
82 300TD
83 300 CD
83 380 SEC