4 x screws keep it together (see yellow circles). I used a normal flat headed screwdriver.
It was very dusty inside as you can see from the next 2 photos.
First of all I was cleaning the tool with compressed air.
Then I used a multimeter in order to check for continuity between the two cables connected to the power switch (yellow circles), with the switch set to "ON". There was no continuity so I pried the switch open using a small flat headed screwdriver. The contact points (red circles)were really dirty.
I cleaned the contact points (yellow circles) using a fiberglass brush (red arrow) and put the tool together again in reverse order.
Another electrical tool was saved from the junk-yard.