If the wiring is mixed up, the sensor or module that the connector is attached to will input/output incorrectly which at minimum will cause issues with that system and at worst cause permanent damage to the sensor or module. Below are some pictures of common plugs and how to read them.
In general, look at the front of the plug (terminal side not wire side) with the locking tab on top and read it left to right for numbering. One additional thing to note is that it is in the opposite order from the back.
With a two-tier plug, the same reading rule applies from left to right and then go to the next row from left to right. It's similar to how a book is read, left to right and then moving down to the next sentence.
Of course it is Subaru wiring so there always some exceptions but for the most part it is that simple! iWire recommends using wiring diagrams for all wiring projects to ensure the correct placement of each terminal. That way their are no electrical pixies down the road. If you need any Subaru connectors, terminals, or seals feel free to Contact Us!