What does this do?
 

Hello..

I have a knob on the dash of my coupe that doesn't appear to do anything.
I have circled it in yellow. It pulls towards the driver with a positive click, ie on/off, no turn. From left: (non functioning) rheostat, the hazard switch, air vent, magic mystery knob, rear courtesy light switch.

The car has no fog lights front or rear, and doesn't appear to have ever had any. It's not a radio fader either.

Any ideas? Thanks.

Dave

Most likely rear windscreen defroster.

Ok..makes sense..didn't think of that because car has no heated rear screen, but screen has been changed. I imagine a '71 3.5 coupe probably had a heated rear screen as standard?

Thanks.

Now you can repurpose the switch for a James Bond ejection seat. Just be sure to get one for a RHD model and not a LHD one from the USA is it will be on the wrong side.

Good point....don't want to make such a potentially fatal error whilst battling the bad guy.

That button also lights up when it's on.

Does it? I don't see how..the white dot is a solid sticker type thing, as opposed to the visible bulb in the knob for the hazard switch for example. I'll have to take a closer look.

On my car (76 280C) the white dot lights up. I'll take a pic tonight when it's really visible.

Here are the pics of said button...

The manual for my 1972 300SEL says that button (which does light up when pulled out) turns on heated rear window. The heat lines are very fine.