I have an 88 Merc 45 hp that won't shut off when the ignition is
switched off. I replaced the switch and that didn't fix it. Am thinking
that there's a wire or something loose in the engine - maybe the
magneto needs to be grounded and that's not happening. Anyone have any
advice? Thanks!

I'll kep plugging then. I think I need to look for a black wire with a
yellow stripe, someone told me. Thanks for your post

You are on the right track. The yellow black wire is the kill circuit.
Have you tried activating the Kill switch if so equipped. The ign.
swithch and kill switch are on the same wire.(BL/Y)

The bl/y wire is attached to the switch box. You can remove this wire
and test it with an ohm meter or continuity tester. One lead to the
wire the other lead to ground. When key is turned off should show
continuity. The other possibility is a bad kill circuit in the switch
box. To test the kill circuit you will need a DVA meter. Usually only
repair facities have this meter. Hope this helps.