The wire for the temp gauge is on the rightside of the engine and connects to the temp sender just aft of the thermostat housing. If this isn't the loose wire you mentioned, turn on the ignition and ground the wire. The temp gauge should go all the way up which means that there's either a poor connection at the sender or the senders kaput.
The horn wiring is on the leftside of the engine bay. When the horn button is pushed, do you here a relay clicking? You might find
this wiring diagram helpful in tracking things down.