Assuming you are running close to a stock style setup, your v-belt driven alternator works just like any other alternator, whether it be v-belt or serpentine driven.
The faster the motor spins, the faster the crank pulley is spinning. The faster the crank pulley is spinning, the faster the accessories are running that are attached to it via belt.
The alternator is one of these accessories.
So, when you are idling, the alternator pulley is spinning as slow as it's going to spin, and the alternator is putting out only a fraction of whatever its amp rating is.
Most factory alternators do a fine job of powering all the electrical accessories (which were not many on classic mustangs), even with the headlights on, heater blowing, radio on, etc. Though, it is not unusual for the headlights to dim a little at idle.
However, if you have even so much as one or two aftermarket accessories on your car, the amperage requirement has gone up yet your alternator output has stayed the same. So, if you have an aftermarket CD player and speakers, or an electric radiator fan, or an MSD box, or any additional pieces that suck up some amperage,than any weak spots in your charging system will be very noticable at idle rpms.
As a followup I just noticed another post of yours detailing your new big sound system.
You DO NOT have an electrical short because your headlights dim at idle. Your headlights dim at idle because you have imposed a new huge load on a charging system that was adequate for a car that came from the factory with a couple idiot lights, 2 sensors, a point type ignition system, and only a handful of light bulbs.
With your new sound system, as people in your other thread already pointed out, you are effectively discharging your battery because your alternator can't supply enough amps for all the new demand on the charging system.
Check and make sure that you have a ground cable from the engine to the chassis. Originally there was one between the right cylinder head and the firewall. You could as a test use a jumper cable, battery negative terminal to a good solid ground on the engine or even to the case of the alternator. If the lights brighten, poor ground.