Scania G410 4x2. My guess is summat like a kill switch? It doesn’t appear to be connected up to anything though.
Remote tacho download.
If it ain’t connected, then it does nuffin’ 
Is it the iscolater 
only a guess
Go faster button for the irish
Is it the automatic frilly curtain positioner and wheel polisher.
anti cyclist tazer control
Didn’t you post this on Facebook we told you it turns tang mode on or off
Door electric charger to stop intruders.
Passenger Ejector Seat ?
Rozzer and Dvsa detector button.Press once and location of the old bill comes up and if the checkpoint Charlie is open for tea and biscuits.
teatime:
anti cyclist tazer control
I think this is the most sensible answer 
Coffee making button.One shot of Expresso.Press twice for double shot.
Or as said,frilly curtain adjuster switch.
OMG you’ve found it. This is the magic button that wakes you up and you realise the whole world is just an illusion and your really in a testube that converts your bodies electricitity into a powersource for a big computer thingy to keep everything working.
Watch out for VOSA agents though as they are slaves to the machine and if they see you press it or attempt to press it you will need lubeing up big time. Anyone dressed in tight black PVC is there to help you so always ask them that you would like to ‘see how far the rabbit hole stretches’ especially in areas such as Brighton and Hampstead Heath.
No prizes for guessing what film I watched the other day…
Or even more far fetched is its a dummy button used by less scruplous hauliers to switch the limiter on and off and hack the tacho so it never registers above 60mph, so MMTM in an RDC once.
Just keep pressing it a lot and see what happens.
seth 70:
Is it the iscolater
Correct i think i may be wrong hard to believe i no but sometimes i am
Its the ding dang do switch that makes u turn and burn so you can get a hollywood finish 
It’s a rocker switch thumb checker / exercise machine.
I`ve seen one of those before, its a Gozinter, and can only be used with a left handed screwdriver.
truckyboy:
and can only be used with a left handed screwdriver.
You just know someones gonna look on ebay for one don’t you
PS you can only get a ratchet sort
It’s an invisibility button for when you’re speeding past a camera. Try pressing it while you’re passing through a 30mph camera on the limiter, see what happens.
Bernard