The proper way to do it is to run a wire from the brake pedal switch to the 3rd light. IF you tie into your tail lights the third will blink when you put on the signal light for the side you tied into.
2004 Chev Silverado Duramax optioned past the max. 2009 Jayco Eagle 308 RLS 765 watts of solar, 6-6 volt batteries (696 amp hour), 2000 watt (4000 surge) whole house inverter.
140 days boondocking in 2016
211/2015, 196/14, 247/13, 193/12