I wired mine right into the fuse block. If you want the gauge to light when the ignition is on, pick a fuse with a constant. If you want them to light only when the lights are on, wire it into the headlight fuse.

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R