after market radio loses power when i step on brake

Who installed replacement unit?

after market upgrade to sound system maybe drawing to much power circut to keep up with. best to get connections checked and alternator tested for output. compare with system draw. it may also be the battery. if battery is not able to hold enough charge to run components, you may need to upgrade to deep cycle to keep up with system draw. double check and test everything related. audio tech. should be able to address as needed.

how long has the radio been in? brand new? ck pwr source they may have tapped into the wrong wire

It was the cabin control module easy repaired a little solder about 15 min .