The electric chokes come in 2 general types: non-switched 12-volt ones and switched 6-volt ones.
The non-switched type start opening as soon as 12 volts is applied to the coil, so it acts like a time delay.
The switched type have either a bimetallic thermal switch or a solid-state material like germanium that does not conduct eletricity when cold. At about 50 degrees (F) this 'switch' becomes conductive and grudingly begins to flow electricity. As it does, it starts to warm itself up rather quickly, depending on ambient temperature, and this heater heats up the little coil that opens the choke plate. If it's really cold outside, it opens slowly - if it's warm outside it opens quickly. This is the ideal situation, but it usually requires extra plumbing in the form of tubes that bring warm air from somewhere into the choke housing so the switch will 'start'. Toyota uses hot water from the heater circuit; Ford uses warmed-up air from the filtered side of the air filter that goes around, through or near the exhaust manifold before entering the choke housing. This system often corrodes away in the 200 I6 because the little heater tube is inside the exhaust manifold, where the MTBE in the gasoline eats them away. (When this happens, the exhaust gasses enter the choke housing and eat the choke and damage the carb.)
Most of the aftermarket types I've seen, like from Whitney, et al, are straight 12-volt types. You just hook these up to the IGN circuit in the car. Most OEM ones are the switched type, and they connect to a special tap on the alternator (7 volts AC) that is designed specifically to operate the choke. You will not usually find this tap on a stock Mustang alternator, so you have to change the alternator.
If you apply 12 volts to the 7 volt types, the life will be greatly shortened.
