All you need for the dash to power up are all the black wires connected to live and all the brown ones connected to ground.
I'd personally run a new earth to the dash with a reasonable sized cable to another point in the loom and a new ignition live - maybe to the ciggie lighter if that is ignition switched - should help a lot.
Nah - just get a couple of lengths of decent cable, scotchlock them onto the fag lighter feed (temporarily with the scotchlocks of course) and run them to the Dash.
Live goes to the black on the dash (2 cables into the dash) and Ground goes to brown on the dash (3 wires into the dash). Disconnect the blacks and browns from where they are currently connected to at the moment though before connecting them up to the new feed.
It sounds like the ground on the supply to the dash is raising above an acceptable limit when the electric windows are being activated resulting in the loss of the tacho.
Yup - you can do - I would personally put a 5 amp inline fuse with the live feed if you're leaving it connected up to the ciggie lighter and solder the connections rather than scotchlok them for the long term.