Just trace the earth wire back - it should probably be connected to the chassis at some convenient point. Wiring diagrams are not always accurate or up to date and only give the circuit - not the physical layout.
I think it unlikely that there's a fuse in the earth wire.
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Sounds more like an internal pump fault to me. What do you want a wiring diagram for? The pump? The vehicle? We know nothing about either. Just follow the cables with a multimeter in your hand until you find it.
If you are talking about a caravan, you are on the wrong Forum.
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