Is that year model correct, a used 06 already?
I went thru the same drill a year ago while rigging my new 04 90 TLRC.
I am assuming you have a plain old tach and separate trim gauge. If you have the multi-function tach & trim gauge, then you're pretty much going to need to buy the Yam harness.
So, getting on with separate tach and trim gauge installation...
You do indeed need some sort of wiring to make the trim gauge work. You will need to connect all the way back to the motor as there are no factory supplied wires to the control box.
Beings you already have the tach functioning, then we have a little homework to do before you go and spend for the Yam harness. (you will find that the Yam harness is rather spendy, so you don't want to flop on the counter and cash up unless you have to)
Take a look under the dash and behind the control box, and see if the tach was connected with a simple single wire from the control box to the tach. If so, then you will need to run three (I think) wires from the sending unit connection plug (which is on the right side of the outboard, buried in the mess of wires) to the trim gauge.
To make my connections, I picked up some little flat connectors to make the connection at the motor, and then ran plain old black outdoor electrical cord from the outboard to the trim gauge.
Hope this helps & good luck with it.