I did.

Note that my audio knowledge is in home and professional recording and
broadcast systems. Car audio is quite specialized and has an almost
unique ability to permanently shred the driver's hearing apparatus and
create real drive-by misery to others. Having gotten this off my chest,
I'll attempt an answser.

From what you've written, I think that the place to zero in would be on
what's sometimes called the "processor loops." In other words, there may
be a signal out point and a mating signal in point. We're talking about
four jacks here.

Some car stereos have this ability -- the idea is to be able to run the
audio from the head section (tuner, CD, etc.) out to an external
processor device (originally, an outboard equalizer). The signal is then
returned into the unit after processing to be fed into the stereo's
power amplifier stages; from there, the amplified signals go out to the
speakers. You may wonder why there could be connections for your unit
since it's already got an equalizer. Well, some people might get talked
into using an echo device -- which nobody ever needs because all
recordings have exactly the desired amount of echo already and adding
more is like (them metaphor in my mind is not suitable for children).

Two methods of switching are used for such a "loop." One is to use one
jumper cable permanently attached between the jacks for each channel.
The other is via a digital switch in the receiver. Since it's 2005, I
suggest reading the instructions carefully to see if you've just gotten
a menu selection mis-set. And, yeah: some of Sony's instructions can be
the worst Japlish ever conceived -- complete with insane names for
functions and explanations that never made sense in the original
Japanese in the first place (My Sony digital camera has a "sports
lesson" mode!!).

Another possible point of signal interception may be in order to
accomodate external crossover networks, "subwoofer" amplifiers, etc., etc.

So, I recommend putting this thing on your workbench, connecting it to a
12v battery and a pair of speakers, and while reading the manual, try
slogging through the menus. I think that this is going to work. You
should have an antenna to test the FM (a short wire will do if you're in
an urban area)and a CD for raw sound. The reason for doing this out of
the car is to see if the unit is, indeed, working OK.

I'm guessing about your unit totally, so I really don't know what you've
got. But if you get sound on your workbench, this eliminates the stereo
itself as the source of the problem.

For speakers on the workbench, you can use just about any speaker ever
made. Home speakers are fine. You'll just get less sound from them. You
won't damage the car stereo using them. However, whatever you do, do not
short circuit the speaker wires together -- even one strand of the wire
touching the other polarity can instantly fry your output ICs.

Always keep in mind that in a car, an honest 5 watts is a hell of a lot
of power! 15 watts will be across the threshold of pain. And it doesn't
make sense to spend lots of bucks on car stereo because you can't hear
the quality over the road noise. In other words, I believe in keeping
car sound systems really simple and save the money for the good stuff at
home.

Another option that occurred to me is that your stereo may just be
what's called a "head" unit -- in other words, the equivalent of what in
high-end home audio has been referred to as a "tuner-preamp." In this
case, there's no speaker power at all -- your unit is intended to work
into a separate amplifier in order to drive the speakers. Hmmm. You did
use the phrase "head unit," didn't you? If so, you _must_ use this with
an external amplifier. A low-powered one should do nicely.

Further, I really doubt that Hyundai put a separate head unit and power
amp in any car. Why bother? It's not needed in any car. So, I'll assume
that the original factory stereo was the traditional all-in-one box with
speaker lines coming out. In other words, there's never been an OE amp
in your car to bypass. Perhaps I'm wrong and somebody else can correct
me here. But if I were Hyundai, I sure wouldn't waste money on this
extravagance.

Finally, if you're not sure about what you're doing, you may want to
hire an installer to get everything right. As I said above, car audio
has always been somewhat specialized. I was just inside an old Motorola
AM-FM car radio that my friend bought on eBay for his 70s Mopar muscle
car -- he paid big bucks for the thing. I did some preventive servicing
on it for him, and I had to marvel at the uniqueness of what I found
inside it, and also the excellent reception and decent sound, still
after all these years. Impressive.

Richard