That is a good point. Does it just happen at idle? Does RPM have any effect? It could be normal, I don't have that car but hopefully some one with that car can say if theirs does that. Anyway, there is always a voltage drop in a wire when current goes through it. Depending on how it is wired that result could be "normal". However, it could also be due to a "poor" connection or ground. Does that have the old ceramic fuses? If so you might try putting in a new one or at least cleaning the fuse ends and the clips. What if you have the fan on high but just the running lights on? Is it mostly the fan that causes the dimming? If so then you want t o see what wiring path is in common with the fan and the dash lights. However, I would do what Arthur said since he knows what he is talking about. Do you know how to do a current draw test?

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra