Modern Vespa

going into an all DC small frame. planning on running an extension cable back to the battery and connecting into power box of the speedo.

black- goes to ground.
Red- goes DC (when ignition is turned on)
Brown- DC directly from battery

Is that correct on the brown wire?

This is how i recently wired up the SIP speedo in my VNA Allstate (12VDC VAPE with battery).

Good question! Sadly my speedo never worked right out of the box so I didnt get that far. I remember something along the lines of it had something to do with the headlight bulb? Possible induction of AC voltage and that calculated revs? It definitely wasnt meant to go to the battery! I tried that with the red wire and it always kept the backlight on IIRC.

I think-

Brown to a headlight wire. may get inaccurate REV measurements. if fed up with that, then install the KOSO Reader and get precise REV measurements (as above).

Red needs to go after a switch as directly off the battery the speedo would always be energized.