I just fixed one of these on a 87 merc 25. If the kill button is red, then the problem is most likely corrosion inside the red rubber button. Remove the rubberized tiller handle using silicone spray and a small screw driver. The red button will now be exposed and you can remove the the red rubberized cap. Inside the rubber button you will see a small half disk shaped piece of copper that makes contact with the two wired contacts to kill the engine. Clean the corrosion off of all contacts and replace the copper disk back in the rubber. Replacing the disk is a bear so take your time and be patient. Kill button works like a champ now
If that doesn't work, just disconnect the wires for the harness (in the engine bay) and pull the switch out. Make sure you tie a bit of string to the end of the wires so you have something to pull the new switch wires back though with...
