IS THERE A BRAKE PEDAL RELAY OR MODULE WHICH MAY CAUSE THIS PROBLEM ?

What is your question?

2013 XTS WON'T START, NEW BATTERY & STARTER. WHEN ( PRESS BRAKE TO START APPEARS) BRAKE PEDAL WON'T DEPRESS. MAY HEAR ONE HARD CLICK SOUND. ALL DASH LIGHTS CLICK OFF AT THIS POINT. PLEASE HELP

What makes this problem better or worse?

1ST STARTED WHEN ASK TO PLACE KEY FOB IN KEY FOB SLOT , VEHECLE STARTED ONCE

How long has your 2013 Cadillac XTS had this problem?

TWO DAYS