Welcome, and congrats on a great boat - ours is a 1995 220 with the 5.7 Gi Duoprop.
You can get service manuals for the motor and stern drive from Haynes (used to be Clymer, Haynes acquired them). The manuals have schematics for all the engine instruments and control systems.
Our trim gauge was also inop when we got the boat last year. Local Cobalt dealer diagnosed the problem as a bad "brain" (ancient analog unit that sits between the sender and the gauge) and recommended against spending $400 replacing it, as they are prone to failure and what you'd buy now is NOS or takeoffs from old boats. Instead, bought a new digital trim gauge ($400) and a replacement sending unit ($150) from Leaders RPM (
https://www.leadersrpm.com/?utm_source= ... MB-service). Dealer billed us about 1.5 hours to replace the gauge, and determined that the sending unit was OK so we kept the new one as a spare, since parts are getting hard to come by for the older VP drives.
Digital gauge is nice. You get a numeric readout (from -5 full down to 40 full up), plus some indicator lights that show when and which way the trim is moving, and red when it's out of safe cruising settings. Downside beside the cost, it doesn't match the other OEM gauges in appearance, and didn't come with a matching bezel so I ended up getting some VDO plastic-chrome rings from GLS and gluing them on all the small gauges so they match.
Knowing the trim position (not trying to guess position by how long the buttons are held down) is pretty important on these boats, so we felt the investment was worth it.