RPO code definitions:
WJ5 - "Value Leader" Parts Package
88I - Interior Trim Color - Dark Titanium
YE9 - Silverado / Sierra Classic SLE
19I - Interior Trim Color - Black / Ebony
GAT - Appearance Package "GMC All Terrain"
SLT - LT/SLT Equipment Package
X31 - GMC 'OFF ROAD' Appearance Package