Re: 1990 90 hp force 18 ft bayliner trim switch
Once you understand the principle, it's real easy. The switch simply directs low amperage SIGNAL voltage to the correct relay. This relay closes and sends power to the trim motor.
First buy a single pole M-O-M (Momentary on, Off, Momentary on) switch, waterproof, of course. You can even mount it on the lower engine pan, just like the Evinrudes.
Since your engine has relays, it is dirt simple.
Pick up a 16-18 gauge red wire from the red terminal on the engine terminal board. Connect it to the center terminal of the MOM switch. You don't even need to fuse it because in the event of a short circuit (very highly unlikely), the relay solenoid windings are so thin, they will burn out before the wire.
Now, since green is down and blue is up AND since the terminals on the switches are usually reversed: using 18 gauge wire tap into the thin green and white wire on the relay. connect it to the top terminal on the MOM switch. Tap into the blue and white thin wire on the other relay and connect it to the bottom terminal on the MOM. ---THAT'S IT! Either switch will now send signal voltage to the relays.
Note the green and white and blue and white may be solid green and solid blue. As long as you tap into the THIN signal wire it's OK. Do not tap into the THICK power wire to the motor.
Note also that if the up button on the switch moves the engine down, just reverse the green and blue wires on the MOM terminals.