My 1989 tilt & trim should be the same as your 1988 system.
I removed it (trim gauge) few years ago and can't remember how or where it was connected, maybe to the control box ?
I'll look through the manual / install guide ... and get back to you ...
99 percent sure it should work with your motor ...
EDIT ...
The gauge is the same style as my speedo, which was original and used with a 1976 Mercury 115 HP motor and was hooked up and working with my 1989 Mariner 115 HP motor.
The tach cable hook up should have been the same with all three control boxes.
From the install manual ...