it is a standard trim wiring harness with 12v 5pin auto-security type relays. You can buy used "2w trim harness" off ebay for johnson, evinrude, force, usmarine, volvo, mercury, mariner, etc. They are all the same. You have to modify them for use with 3w trim motors. If you try to use a 2w relay harness on a 3w motor the result can be smoke and flames because it can create dead shorts across your trim motor windings.
A relay is a remote control switch. Your air conditioner at home uses them so that that little thing on your wall can turn on that huge thin beside your house. How dangerous to run a 60Amp 480V circuit into that tiny little thermostat. So they use relays. Same deal with trim. little skinny control wires from the switch back to the relays. big fat power wires in and out of relay to motors.
you can have as many switches as you want all over the boat. when any one of them provides 12v to the light blue control wire it activates the "up" relay. when green then "down". each switch and wiring needs minimal amps and fusing since it only activates a relay. You could daisy chain 10 of them off a single run of 22awg 3c. the actual power for trim motor is pulled from the starter and protected with a 20-30A inline fuse or breaker. The relays are typically bolted onto the starter bracket. motor ground is typically bolted to the transom clamp or engine block. trim motor green and blue is typically 36", protected with a sheath, and runs directly to relays. the entire relay harness with 2 relays should run you about $30. Any SPDT momentary switch will work. waterproof switches preferred, about $6ea.