I imagine it will earth through the hinges at the bottom, but its good practice (and stops you scratching your head later when you get an issue) if you run a seperate earth, when I mounted spot lights on my roof I ran a section of 2.5mm twin core and earth which comes flat and is what you use in house wiring on a 240v socket ringmain, this I glued to the inside of the windscreen surround, at the top I fitted a connector and ran the rest of the wiring to the spots in flex, with the connector you can disconnect it whilst removing the soft top & it makes the job a bit easier! The twin core & earth (that was glued to the inside of the screen) I painted black & powered from a relay under the dash, it doesn't look overly bad even if I do say so myself!
A picture of my switches
And my wiring, you can see the connectors
I have to add.... its a nightmare trying to link to Photobucket images on your phone!