Where is the constant voltage regulator located on a 1965 Ford Thunderbird?

A 1965 Ford Thunderbird doesn't have a "constant voltage regulator" in the same way modern vehicles do. Instead, it uses a voltage regulator as part of the generator (or alternator, if it's been converted). This regulator is integrated into the generator itself. You won't find a separate box or unit. The generator is typically located on the front of the engine, driven by a V-belt.