after market radio loses power when i step on brake on 1999 Pontiac Firebird

sometimes comes back on when i restart,

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3 answers
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.
Who installed replacement unit?
how long has the radio been in? brand new? ck pwr source they may have tapped into the wrong wire