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I really hate being contradictory, but I beg to differ. The shifter indicator light is not (supposed to be) controlled by the dash light rheostat. The green bible says so and I've never seen otherwise. The symptoms described are indeed a bit bizarre, but explainable if you look at the wiring diagram hard enough.
The problem boils down to the fact that the rheostat is on the ground side of the dash and tunnel lighting, not on the supply side, plus the tunnel lighting such as the shifter indicator, that is not supposed to be rheostat controlled goes back to ground through the back of the rheostat. It's basically a bad design. The circuits are not sufficiently isolated. It was fixed in the 900's.
Start by checking fuses 21 and 22. If 22 is blown then the grey wire from the shifter illumination light is pinched or chafed and shorted to ground, but if the wire is severed and shorted to ground then fuse 22 may not blow.
The story goes like this. If the wire that's supposed to supply voltage to any of the tunnel lighting, in particular the shifter illumination light in this case, is shorted to ground then, rather than getting its voltage supply from fuse 22, that light will now get its voltage supply from fuse 21 through the (wrong) side of the dash rheostat and the result will be that the tunnel light is now dimmable by the dash rheostat, but will operate in opposite brightness, just as you describe. Simple as that.
So, follow the grey wire back from the shift indicator lamp. The pigtail is a delicate wire, so I expect you'll find the problem there, but you may need to trace that wire back and through a connector. I'm betting you'll either find it broken and shorting to metal, chafed and shorting to metal or pinched between metal or have a screw driven through it.
If I'm right, I expect a thumbs up on this post. LOL
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now
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