1990 f150 ?
 

could someone trll me what the red switch is just below the dash on the left hand side right above the knee ?

Can you post a photo of the switch?

If your vehicle is a four wheel drive truck with manual transmission it may be a clutch/starter engagement override switch which allows the starter to work without pushing in the clutch.

Or, it could be dual fuel tank switch.
Something to do with a towing package.
Accessory lighting.
Aftermarket stereo amp etc power switch.
Anti-theft and/or alarm switch.

Hopefully someone who has a similar truck with such a switch will post what it is. I'm more or less just shooting arrows up into the sky.:D

maybe it,s the light telling you when it,s in 4 wheel drive !

no 4 wheel drive and no dual tanks .. the switch would be right above the left knee next to the wiper button

would it be for the windsheild washer fluid