My rear light turns on when I press the brake or tails are on.
ok the manual says a rather long way of testing the little box that runs the light but its way up under the dash. It works like this though. The brake light switch on the pedal connects and sends +12 to the light box. The box has its own ground and a +12 for the dash light. It then has a +12 the goes to the tail lights. The unit must sense 27 x 2 + 18 watts or 72 watts for the light to not come on. (27 for each tail bulb and 18 for the center light) The key thing here is the 72 watts. If by accident you have a lower watt bulb or the grounds in the tails are poor you will draw less power and have less light output and the REAR light will light up. So check each bulb and make sure the two tails are 27 watts and that the central bulb is 18 watts. Also check the ground wire for the unit itself but I doubt that is it.
2006 Mazdaspeed6 6-spd 274/280 stock.
Those people who think they know everything are a great annoyance to those of us who do.