Cigar lighter bulb circuit, switched + turns to ground when key ignition off, why?

[MisterCruiser]

Could someone explain why the switched positive wire of the lighting bulb circuit on the cigar lighter turns into an earth wire when the key is turned to off position or removed.
The cigar lighter itself has 2 wires and it's bulb lighting the ring around it has it's own different 2 wires. Both these bulb wires have continuity to ground when the key is removed. However, when the key is turned to acc position 1 of the wires becomes 12v.

My question is why does the switched 12v wire become a gorund when switched off?

This is on a 90 series. I thought there was a short to ground on the switched 12v wire but before I started to dive into the dash I said I'd check my 120 series and it's cigar bulb circuit behaves the exact same way. So it looks like it's meant to do this and so it seems to be a legit Toyota wiring method.

Can someone with more electrical knowledge than me explain how this scenario works, and why you'd want the switched 12v to turn into a gound wire when off, please?

 

[Towpack]

Are you testing the + wire while still connected to the bulb?

 

[MODVRS]

Presumably it grounds through the bulb filament? Take the bulb out and see if it still grounds

 

[MisterCruiser]

I just disconnected the 2 pin connector and tested at the loom side of the connector, so the bulb shouldn't have had any effect.

 

[Towpack]

If the fag lighter lights up with the sidelights there'll be other bulbs in the circuit giving a path to earth.

 

[Shayne]

I’ve no idea at the why but when I fried my ciggy lighter it took out the interior lights and central locking if I remember correctly . “Dome” fuse blew so I guess they all connected somehow ?

 

[MisterCruiser]

If the fag lighter lights up with the sidelights there'll be other bulbs in the circuit giving a path to earth.

It does indeed light up with the side lights so they're are other bulbs in the circuit.
I'm confused though, why do the other bulbs cause the switched 12v wire to have a path to earth when the lights stalk is turned off? Are the earth and 12v not separate wires even at the bulb holders contacts?

Ok, I think I get it now. The bulb is in contact with the + and - wires even with the circuit turned off. So the earth wire continues through the bulb into the positive wire. Is that correct?
Cheers dude, I had been pondering why this scenario existed for days

 

[MisterCruiser]

I still don't really understand this, are we saying it grounds through the bulb filaments. So to check this, I'll need to remove all 4 side light bulbs and check then. How can a 12v become an earth though and then for it to revert back to being a 12v when the lights are on. I can't my wee head around how one wire can be both an earth and a 12v though!

 

[Towpack]

Yes, it grounds through the bulb filaments.
The sidelight circuit also lights the dashboard instruments and switches so many bulbs to remove!
The +12v supply must have a path to earth or no current will flow.

Imagine you are dealing with one bulb, one side is connected to earth and the other to +12v via a switch.
With the switch off, the +12v disappears but other side of the bulb filament is still connected to earth which your test meter will see on the + feed wire as a short to earth but with a certain resistance (the filament) depending on bulb wattage. Typically, a single 5w sidelight bulb (not LED) will show around 33ohms across the filament.
Removing the bulb from the socket effectively disconnects the earth from the +12v feed wire.
With multiple bulbs on a circuit the +12v feeds are all commoned together at some point after the switch, hence many paths to earth.