The 4WD actuator on a 1999 Chevy Blazer is located
on the passenger side of the transfer case. It's usually fairly close to the transfer case itself, but its exact position might vary slightly depending on the specific trim level and options. You'll likely see a linkage connected to it and possibly some electrical wiring. It's often a relatively small, cylindrical or slightly rectangular unit.
To be certain, you'll need to consult a repair manual specific to your 1999 Chevy Blazer. A diagram showing the location of the transfer case and related components will be invaluable. You can often find these manuals online (sometimes for free, sometimes for purchase) or at your local auto parts store.
