Audio Systems: stereo doesnt work when headlights on, hyundai elantra, tow truck driver

Question
History:
I have a 99 Hyundai Elantra.  About a year ago, the battery
died because I left the lights on.  A tow truck driver tried to
give me a jump start, but he jumped it with a booster box
with the cables reversed - Twice!  

Obviously the car didn't work and had to be towed to the
dealer where they replaced the resultant blown starter fuse.  
Too bad I didn't play the radio for two days; because when I
did, it had power (the LED lit up), but didn't work.  No one
wanted to admit it was from the same event, likely so they
would not have to pay for it.
 
NOW:
This week I took the car to a PepBoys to get the radio
replaced (since I didn't want to bother figuring out how to
pull and replace it). They had a novice  (covering for the
guy who "called-in") replace the radio with a stereo/cd
player. Without the lights on, it worked fine.  With the lights
on, it worked, but only when the dimmer switch was turned
up all the way.  Otherwise, there was interference which
blocked all recognizable words or music. The signal/sound
also went in and out on a very bumpy road.

Because of the above, and because of a weak signal with
the newly installed player/receiver, I had them try I again
with an upgraded device (a Pioneer of some sort @about
$110 or $120).  I told them about the lights, but I can't be
sure the installer knew, because he was gone when I  
dropped off/picked up the car.  I suspect he just hooked up
the player to the other guy's wiring and added a pad to
stabilize the player.  The new player sounds better and gets
a better signal, but doesn't work at all with the lights on,
regardless of dimmer switch position.

When I speak of lights, I mean the headlights or parking
lights. The radio plays when the interior light is on, or when
the wipers, or fan, etc are on.


So now I'm taking it back, but I fear they may be lost with
the lights scenario.  Any suggestions?  Please and Thank
You.
Sincerely, Mike
P.S. The factory radio did have a blown fuse in the back - I
presume from the jumper incident.

Answer
Hello Mike,
I can suggest that you take both the Hot wire and the memory wire from the radio and twist them together (temporarily for testing)and then hook them directly to a 12 volts at all times source. After you do this turn on the radio and see how it plays. I beleive the installers may have connected the radio to the dimmer control lead. With the key on the dimmer lead will have 12 volts on it, however when you turn on the head lights the dimmer lead voltage will vary according to the position of the dimmer control. On some cars the dimmer circuit voltage is a pulsed on/off voltage to allow the dimmer circuit to function without heating up. if you have a radio connected to this line even if the dimmer control is turned to full brightness the radio will not work! As for the reception going in and out on bumpy roads, the cause may be a defective connection in the radio or perhaps a faulty antenna connection.

TOM,
T&D ELECTRONICS
tdeser@zoominternet.net