Yellow wires are typically for the charging circuit on a merc (they are on my
2002 60 4 stroke and my old 89 75hp)

The easiest way to get a rectifier working is to buy the merc unit ($30-40) but
it is basically just a bridge.
You could use one of those square block diode bridges if it was good for 20a or
so. The 50a ones show up on surplus sites pretty cheap.