Depending on your options you will have a few connections that may not have a “home” to plug into. If the brake light was not on before the double din radio work I would suspect might have bumped the emergency brake a notch and tripped the light on. If you had accendently unplugged it no light would occur. That said easy check first, look at brake fluid level under hood. Low fluid will give a indicator light. How low your brake pedal sit. WHen the pads wear down a good amount the pedal general sits lower and fluid drops. Lastly, the usual- normally the pads have metal tabs that will rub on rotor making a squeal sound to make the driver aware the pads need replacement soon. So- check your stuff, brakes are a working in good order necessity.