Rear brake lights aren't working.

I have a 1997 Nissan Altima that looks to have an aftermarket stereo in it.
My electrical in the car doesn't seem to be working right.  While driving with
my lights on, my instrument panel, and my stereo will cut out for a few
seconds sporadically.  I believe that this is related to my brake lights.  

I first noticed this issues a few weeks ago.  When the lights were off, My
brake lights would work except for the top light.  When my lights were on
however, the brake lights would not get any brighter.  Today, I had my car
inspected, and it has since stopped doing that.  Now, they won't turn on at
all.  The rear lights themselves turn on, but no brake lights.  There is one
light on the fin that they said was an LED light that does come on in both
situations, but none others.  Is this an electrical issue?  Or a lightbulb
issue?  I have a little experience in auto repair, is this something I'm likely
to be able to fix, or should I have a mechanic take a look?