I would first check to see that the trim sender & wiring is in place . . .
Starboard pivot point of the outdrive for the sender and the wiring comes through the transom assembly and is probably tied off at the rear of the engine somewhere.
If that is all in place, you will just need the trim gauge (any after market that is intended for Mercruiser will do, but if you can get one that is a good match to the rest of your gauges, even better)
You will want to review the layout of your instrument panel and the space behind it to see where you can best place a gauge in the instrument cluster.
Your boat may already have the wiring for the gauge, so you will want to check that out by reviewing the wiring diagram for your engine and see if there is an unused wire of the correct color coding (brown/white ???) at the rear of the engine and also at the helm. If so, then you are in good shape. there should also be a ground wire at the rear of the engine that is unused, as that will be for the sender as well. The connectors are opposing male/female bullet style.
You would then only need to connect to the +12v and Ground wiring from another gauge at the helm, using some 'piggy back' connectors.
The hole you drill for the gauge will most likely be a 2" hole, but verify that once you have the trim gauge.
So, those are my comments, other may have additional comments as well.
