Ammeter heat question

Lots and lots of variables
1...Wiring is old, and under that fancy molding around the wire eyelets are crimp connectors, subject to damage from corrosion and heat damage
2...The wiring was always marginal, and if you've added higher wattage headlamps, stereo, big alternator, etc, the load is increased.
3...Even if the wiring is OK, "it might be" that oxidation on the eyelet surfaces + the type of bolt (plated) reduces conductivity even though connection is tight

100F is not hot. Barely above human body temp.

What you REALLY must do MINIMUM if you want to bypass the ammeter

1...Yes bolt the ammeter wires together BUT!!!
2....Inspect and repair the bulkhead terminals. Absolute MUST
3...Run a bypass direct from battery to alternator through a fuse or breaker. Depending on alternator output, use big wire, no8 min. is good for up through 60A, or parallel about 3 no 10 wires

"One wire" alternators must use much larger oversized output wire, because the sense circuit is over the same wire, and voltage drop becomes very important