Hi Gipman. The only Sun Country repaint I can find for the FS2002 default 737 is the one posted on this page.
http://www.simviation.com/fs2002jets1.htmAssuming this is the file you're having problems with, please read my walkthrough & follow the instructions carefully.
http://www.simviation.com/lair/Repaint2002.htmI tried it for you following my own instructions. The new texture is named b737_400_T.bmp. This is the result.

Please note that your new Texture folder can be any name you wish - providing it follows the FS folder naming conventions using a period - Texture.3, Texture.Red etc. The name of the folder after the period is defined in the texture= line of the new CFG entry. I named mine Texture.Sun with the corresponding entry
texture=Sun
I hope this is some help.