Lights dimming when using the windows shows signs of a weak battery,mine does it and I know my battery is on its way out.bonnet earth is not that important so don't worry about that to much,any earth strap will work, underneath the battery tray there is 2 or three main earths and theres another bolted to the bell housing and one on the coil to head,think the mirror glass just prise off ,can you hear it working and no movement as could be the switch.