Hi welcome along worth checking weather strip for cracks damage can remove just be careful clean gutter area after where crap collects I used a product rubber conditioner to keep it supple I had a small rip in mine repaired and refitted wasn't seated properly that solved it for me.
Also worth removing tail lights see if any signs getting in round seal there.
Is there a spoiler on your impreza if so worth looking there too factory seal on boot lid break down round em after years check under spoiler clean inspect when remove too
Good luck