It's indicative of an intermitten bad ground, Vic. Some ground connection for the circuit the dash lights are in is intermittently high resistance; when that happens, the current that usually flows from the dash lights to ground can't, and it finds its way to ground through another path, the turn signal circuit, thus causing those lights to dimly illuminate.

Repair involves making sure all the affected ground connections are clean and tight. I would remove the dash cluster, and clean *all* its connections thoroughly. Behind the dash and within the center stack there are several (two? three? four?) ground connections, where a bunch of wires come together under a bolt or sheet metal screw into the car's structure. You'll have to find these points - I think Bentley has a diagram showing their locations - and disassemble, clean, and reassemble each of them. When the grounds are clean and tight, the symptom should disappear.
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David Brick, Santa Cruz CA, 1988 245