Is there any issue with splicing heated grips into the same wire that provides power to the speedo/tach? Will it provide enough juice? Its already cut due to a previous venture so it would be a convenient place to tap..
If you are going for the splice technique I would be going for the headlight power supply. That way it will be switched with the ignition and should handle the power fine. Grips tend to draw up to 4A, well I run two headlights and it copes with the other 4.5A headlight fine - so I can't see any problems there.