Maybe just some sort of thermostat, a bimetal and a thermometer... How accurate does it need to be?

Tar&stuff.

Possible radiation caution.

Tar&stuff.

Confused, these are LEDs, a silicone diode is not ( don't know what LEDs are made of I must admit). Is this a trick question? LEDs tend to drop about 2v, different types different drop. Need more spec, don't understand spec stated, where did you get them give me a component number or link. Do you un...

Get a multi meter ... Maybe a cheap electronics kit, ie a bag of components and a bread board, maybe srip board, a few transistors, resistors, LEDs, 555timers etc, and start again.
It will be easier on second reading.

Tar&stuff.

Hi, I would recommend a cheap digital multi meter with DC 10 amp function, you don't need 10amp for this project but I recommend 10amp function because a multi meter can be used for other projects, and you will probably regret not getting 10amp function at a later date and have to get another meter....

You need other circuits to go with it but it could be done. In the bit that chimes there are circuits which can be tapped into, when you push the button instead of switching a chime circuit, it switches maybe a flipflop which powers a relay, maybe via a transistor and resistors, which switches the l...

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...

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.

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

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

Tar&stuff.