We got the same tach with an electric temp guage as stock one stinks. We wired the backlites to the instrument cluster lighting - to the dimmer switch. So when the lights are on - so are the backlights for autometer guages, when you dim instrument lights gauges dim as well.
We tied the power together so we didn't have to run multiple power lines - keeps it cleaner as well. You're correct with wiring to fuse panel; wire to the fuse side not the always on hot side. We picked a fuse that's only on when iginition is on.
If you have a volt meter it's easy which side to hook into. Put your - lead to a good ground; pull the fuse and test both sides. The side that's dead with iginition on with no fuse is the side you plug into.
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