[Brainchild]

Check where the wires run under the door sill plate,could have a screw down through a power wire grounding it out.Also the brakelight switch may be going bad and leaving the brake lights on.
If it just started this upon installing a new stereo I would guess that this is the problem.Have a look to be sure everything is hooked up correctly.

Good luck

I hate wiring <shudder>