An easy way to update the look of your Kodiak/Topkick is to swap out the tail lights for a more modern set.  While there are lots of after-market options out there, newer 2004+ Chevy lenses and harnesses provide the mature look and superior quality you expect from an OEM product.  To correctly complete this swap you will need both the tail lights and the wiring harness.  Both can be purchased at any dealer or can frequently be found on many of the on-line auction sites.  

Start by removing the two philips screws highlighted above.  With the two screws removed the tail light will pull out.  With the lamp free of the vehicle, follow the harness back to the electrical block shown below.  While under the truck cut any zip-ties that secure the harness to the frame and remove any other fasteners that prevent its removal.  

The tail light harnesses are shown above.  They are the two black connectors located in the center of the block.  The lower one is the passenger side with the upper connection to the driver side.  Take care not to swap them as they can be plugged in either way.  The longer of the two harnesses is the passenger side.  The harness can be unplugged by depressing the center tab while at the same time pulling back on the harness.  Plug in the new harness and fish it back to the tail lights.  While under the truck go ahead and reinstall the harness retainers and replace any zip-ties you may have cut.  

The only modification that needs to be made to the lamp housings themselves is to trim the upper alignment tab on the housing.  The mounting hole on the Topkick is shaped like "-" while the alignment post is "+" shaped.  Highlighted above you can see the first two of the 4 cuts you will need to make.   To remove the top and bottom of the "+" simply make two cuts as shown above using care not to damage the front of the lens.  With those complete make two horizontal cuts to remove the excess tabs.  The new light should now slide right back into place.  Before finishing install the four bulbs and test fit the assembly.  When everything lines up, reinstall the two screws.