Just open the mirror covers (this is the hardest part be careful not to snap the fixings as a bit of force is required) Inside the mirror is another covr that houses a microswitch that is activated by the mirror folding. The microswitch lever has probably come away from the microswitch. You just clip the metal lever back on to the microswitch body. You will have to check both mirrors, as both need to be functional for it to work.