This was a concern of mine, as was the fact that the existing installed
wiring was only rated for 5amps.  We used the Hidden Hitch brand wiring
adapter/transformer.  It was only about $50 installed and has its own main
power wire.  We spliced into the Kia wiring plug but the only thing that
does is "trigger" the solid state relays in the adapter.  Our trailer lights
are plenty bright and I get full voltage at the socket with my meter.  If
you look under the truck on the passenger's side behind the rear tire, there
is a plastic cover that protects an empty space.  This was the perfect place
to install the adapter and extra wiring.

In the long run, I spent just over $150 + tax for the exact same thing the
Kia dealer wanted for over $400.

As an added note, Kia has stated that the max towing capacity of 3500lbs on
the Sorento is limited by the factory hitch.  With the Hidden Hitch, the
capacity increases to 5000lbs (not that we would be towing anything that
heavy, but it's nice to have).  If Kia starts using a different hitch, you
should see the listed capacity increase in future model years.

Cheers - Jonathan