Electronics 2000

No wait, the satnav is mono, one of the satnav wires is earth.
I will av a fink.

One wire from the satnav goes to both inputs to the relay... The other satnav wire is earth.

Tar&stuff.

Maybe consider a cap between the relay outputs ...not sure what size...wild guess... 1uf.
May be try it as it is and if it crackles on switch over suppress it with a cap.

Tar&stuff

I understand that one of the sat nav wires is ground, just split the signal into 2 to go to L and R, if thats ok then i will try it and if it crackles I will suppress it

If this works could commercialise it and make a fortune (just joking)

It will work if the relay coil has a high enough resistance for the 555 to handle, if not you can add a transistor, the worst you can do is blow the 555 which is pennies to replace.
You might need a resistor on the satnav output so the speaker resistance is right.

Low batt back in a bit.

I said speaker, obviously the relay output goes to the aux input not the speakers, I had a low batt and no time to think.
As doubling the satnav wire to 2aux inputs reduces the resistance to the flow of current from the satnav the satnav will drive more current than in mono.
I would say to be on the safe side mono is safe, mono to 2 aux inputs might damage the satnav. I should think it would be OK but then again I can't say it definitely will be.
A way round this would be feed the satnav output to another device such as a a radio or amp or something and double the output from that to the aux input, if anything fries it will be the new device.

I should think it would be OK any way but would not bet my life on it.

Tar&stuff.