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Likely the fuse isn't designed for the higher draw? Satnav would pull at least 2amps (if you can have a look at the back of the satnav, it should give some info like volts and also the amps it uses). What size is the fuse that's being used? It maybe 5 and the satnav could be pulling 2-4, but because the wiring is quite old, there's likely more resistance in it (corroded or frayed wires) causing the draw to be higher.
Other thing it could be is that the wire is touching ground somewhere near the connection to the lighter and by pluggin something in, it makes contact to ground.
The fuse in it is a 10, not sure if that's the right one but it's what was in there.
I've since seen some people recommend altering the shape of the adapter as that could cause problems, not sure how. Others have said they replaced the whole socket which might be an option, on the roads up here in the Yorkshire Dales/Cumbria I like to have proof it's the other idiots on the wrong side of the road should a bump happen