From the
GM Upfitter Guide (Page D-17)discusses electrical best practices for crimping and splicing.
3. Crimp the splice sleeve on each end. Each wire must be crimped individually. For proper placement, see Figure 28. (Note: Use the appropriate crimp tool designed specifically to use with both crimp-and-seal and butt-splice sleeves.)
The method we discuss in the blog uses a high quality 3M splice with marine type heat shrink to seal the joint.