Re: power tilt / trim wiring harness
That's the one, although you may not actually need all three buttons depending on whether or not your trim limit switch (usually on the port cylinder) is functioning.
As designed, the bottom and middle button are for trimming the motor - the bottom one lowers and the middle button raises. Once you get to a certain point, the trim limit switch opens and it won't raise anymore. To completely raise the motor up and out of the water, you press the middle button AND the top button at the same time, effectivly bypassing the trim limit switch. If you do buy a used three-button switch, check that someone hasn't bypassed the limit switch.
Some people don't like the limit switch (or it doesn't work) and bypass it. If want to bypass it or your limit switch is broken, there is a Mercury rocker style switch or you can find your own switch.
Shaun