Why would all electric systems flicker in my car?

[bricmpt2012]

'94 Camaro, V6 auto, 125k. All electrical systems (interior lights, headlamps, pressure gauges, etc.) flicker off and on constantly. Been this way for months and it is really getting on my nerves. Had a shadetree mechanic try to attach a code reader and he couldn't get a reading. Please, no wild guesses.