Put an ohmmeter on the ign switch. It is likely the two MM poles are connected in the off position. If so, one gets ground and the other Ign Kill (black/yellow)for an ADI Ignition (non-dissy)
Turn the ign switch to the on position. See if the b pole is connected to the A pole. If so, A is +12VDC for gauges. If you have a dissy in that Merc, the A pole gets connected to the white wire that runs to the switchbox as well. S is likely starter solenoid. Check to see if S gets power only when the switch is in the start position.
If you just guess, or rely on a multi-engine manual (Seloc) you can easily burn something out.