How much current will the radio use?  If it's not too much, you should be
able to wire off the interior fuse box.

If you must wire to the battery, look at how the vehicle harnesses are
routed.  You can put a hole in the grommet and snake the wire through
there.

Getting through the firewall on a Santa Fe isn't exactly easy.  I
disagree with Hyundaitech's suggestion of using the interior fuse panel.
 I believe its feed line is only 12 gauge or so, which may not be a
limitation for power but it has a much higher chance of being noisy.

I found the easiest way to route a wire from the battery to the interior
into my '01 to be route it under the drivers seat (I think theres also
one under the passenger seat but I didn't want to 'lose' my trashcan).
Theres a rubber drain plug under there.  I cut a hole just big enough to
pass the wire through then once I had everything routed the way I liked,
I made sure it'd *never* leak by using some old black windshield sealer
cord which is super-duper sticky.

The only thing you have to be careful of is to stay clear of the
steering/suspension and you don't want the wire rubbing on the
transmission housing.  I used at least 20 nylon ties in the process.

I used 4 gauge double insulated (black over yellow over copper) "welder
wire" from tractor supply company which was massive overkill for my load
but cheaper than buying 8 gauge from anywhere else.  Plus the welder
wire is very flexible for easy routing.

And of course sure to put a fuse as close as logically possible to the
battery.

In reference to your other question concerning the power seat circuit.
We also have an 03 LX with heated leather.  I've had the center console
out to clean it and I did notice that none of the wiring going in/out
the switch was very large considering the sizable fuse on the circuit.
I want to say no larger than 14 gauge - kinda sketchy if you're running
a 50W+ transmitter.

JS