Despite what some people might say a bad earth will not make two independent electrical systems cross-over - a bad earth will stop something from working - it will not start something working that should not be working. Electricity is not like that.
Somewhere in the vehicle there is a crossed connection - when did the problem begin and what work was done immediately before the problem started - are good questions to ask.
The break light earth is good else the break light would not illuminate - the indicator earth ditto.
Happy hunting...
Peccavi
November 2009