That's an interesting temp line, but I'd be willing to guess that it is aftermarket along with the gauge. Check the manual on what reading that specific temperature sensor supports as it may now be disabled to support the new temp gauge.

I checked my 89 and confirmed that the line coming off the vac canister is only a drain and is alongside the overflow tank drain for the radiator.

The ground cable routes from the frame connection to the lower rear bolt on the AC bracket. I'm not 100% that is the factory configuration, but it was on mine when I removed the motor.