Ground the sense wire, if gauge fully deflects and is steady replace the sendor in the fuel tank.
Trim gauge not working is probably broken wire on sendor. Puck on side of gimbal ring on outside transom assembly. If your bellows are due to be replaced this is the time to replace the sendors otherwise wait until bellows need replacement
1997 2400 SCR
5.7 Vortec / Bravo 2