After Market Ammeter?

As I have the car's instrument panel out, the wiring diagram shows a 12 gauge red with tracer going to an accessory circuit breaker. This wire was not on the ammeter. Anyone know what the deal is on this??? Thanks

GRRRRRRR, You didn't say what car or diagram you're looking at.
Not every build will have every wire shown. If the car came with towing package or some other accessory there would be accessory circuit wiring for that, trailer brake or whatever.
Most of us don't even have a clip in the accessory port to fasten that fuse in.
Just for the sake of saying.. when a rally dash got a tachometer that accessory was wired through accessory fuse. When the rally dash got a clock that accessory was wired directly from amp gauge to it without a fuse.