I believe there are a couple of parts to it, the front panel (which replaces your dash clock) and a computer box that does the calculations. They were available for the E series models, and those have different parameters than the I computers because of the difference in engine/performance. I believe the computer then hooks up to the dash display or SI board, but I'm not real certain about that.
I'd like one myself, all I have is the clock, and at that, it doesn't light up anymore.
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Changed the car's name, now it's - Roxanne -1985 325e