It would help by first telling us what year and model 4runner you have
Ahhhh....
Sorry about that. I have a 2014 trail edition.
So, me being like this :ybrick: was fixated on working at it from the front of the railing than the rear since it's easier access from the back seat doors...
So, I went ahead and popped off the REAR plastic caps for the rails, and the cross bars slide in no problem.
SO... for future reference, anyone interested in installing the factory crossbars...
Step 1. Think. Remove the rear plastic cap from both sides of the railing (left and right). I used a fine tipped flat head screw driver.
Step 2. Mount the crossbar end brackets to the crossbar. Loosely affix the end brackets with the supplied bolts, do not tighten them or it will not slide in the railing easily. If everything is loose it slides nicely.
Step 3. Ensure you install the front crossbars first, followed by rear. Then, if you are not immediately mounting anything, line the crossbars up as indicated to the predetermined spot which will reduce wind noise and vibration as much as possible.
Step 4. Tighten all the screws. 4 total. Hand tighten the crossbar adjustment screw.
Step 5. Replace end caps.
Step 6. Post how you figured it out upon removing head from ***.