The difference in my approach is: I did it without an additional switch. So the mouse is not cutted and the mod cannot be noticed from the outside. It is a clean mod.
But the circuit does need a button to activate the rapid fire, i used one of the existing buttons of the mx500.
See the pictures.
Step 2: The button used
I desoldered it and interrupted the circuit points with a piece of electrical tape.
(The red piece on the left side)
The two soldering points on the right side of the button were originally used by logitech for keeping the button in place. I used these to connect the wires. One wire is coming from the rapid fire circuit, the other one is going to the left mousebutton switch. (See next step)
Step 3: The switch
The cable comming from the rapid fire button is connected to the pin on the right.
So when the rapid fire button is pushed, the rapid fire signal goes straight to the out pin of the left mousebutton.
Step 4: Summary
My little instructable ends with some pictures just showing the circuit which is explained elsewhere.
Ok this was my first entry on instructables.com, i hope you like it! =)