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"Side Marker Light" just means a little red or orange clearance light you would see on a trailer, or on the running boards of a "cowboy pickup truck". 
Folks use them for this application because they are a cheap, and readily available lampholder for a #194 lamp bulb, used in the alternator's "switched excite" circuit. You can mount it out of sight under the hood somwhere, or mount it so it is partially visible beneath the dash board. OR, you can find a pilot lamp assembly to mount in the dash, and have a true "idiot light". The lamp used in the pilot light assembly must have current draw equivalent to a # 194 lamp. OR, you can use a 10 Ohm, 10 Watt ceramic power resistor in place of the idiot/side marker light. OR, you can do it the simplest way of all: 
and use a diode, as shown here. A 1N5008 diode is WAY OVERKILL for this application, yet, is cheap and reaily available. The white banded, cathode end, goes to the alternator's #1 terminal. The diode is shown bare for illustrative purposes, only. I cover them in a double layer of heat shrink tubing to insulate the diode leads, and to prevent them from flexing, and breaking from vibration.
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