got a multimeter? i would check voltages:
*cigarette lighter
*headunit main powerwire (yellow)
*headunit ACC powerwire (red): may be connected to same wire as yellow
they should obviously all be +12V or there-abouts. seems like yours are dropping down significantly.
possible causes include:
*fuses: check/replace all relevant fuses under the dash
*ground: as above
i presume all other electricals seem to work ok in the car? do the dashlights dim at all, or any issues with power windows/mirrors, etc?
