The 18 ga red wire connects to the brake filament of a two filament bulb which does double duty for both brake and tail functions.
There are two filaments in an 1157 bulb, and two buttons on the base, allowing the filaments to be independently energized. The fat filament is used for brake lights and is driven off the 18ga red wire as described. The longer skinny filament is energized by the headlight switch for tail lights through a different wire.
This brake/tail bulb is the outboard most lamp of the three lamps on each side, and sits behind a red lens.
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