The E21 emergency flasher switch is a spring-loaded push button that's under tension constantly; eventually the plastic parts of the switch button break & the button can't detente to an 'off' position, thus leaving it stuck on. You have to pull the switch out of the dash & replace it. I do forget how it comes out, my E21 breathed its last in 1991 so it's, uh, been awhile to say the least. In the meantime you could probably pull a fuse but I don't know offhand what else would be in that circuit.
