You can run your own wiring if you have the proper snake tools. Once you pop the plastics off, wiring can be snaked down any side of the hatch.
If you are looking to wire the rear wiper from the GTS in to your GT, there are a few other things that are needed before it will work as expected, such as a switch to turn it on/off:
The center stop light / third-brake light on the top of the hatch, not spoiler, should already have wiring
https://celicahobby.com/gallery/pic...3-toyota_celica_parts_catalog_by_sectionand
https://celicahobby.com/gallery/pic...3-toyota_celica_parts_catalog_by_sectionClick the image for the first page of the repair manual backdoor section: