I'm always hunting for those specific crossovers where William Afton's chilling presence bleeds into the digital horror of Glitchtrap, and it's a niche that requires knowing where to dig. The most direct route is through Archive of Our Own; their tagging system is a lifesaver. You'd want to filter within the 'Five Nights at Freddy's' fandom and then combine tags like 'William Afton Springtrap', 'Glitchtrap', and 'Crossover'. Don't just rely on the crossover category tag itself—sometimes writers tag the specific elements instead. I've also had luck on FanFiction.net by searching for stories that include both 'Springtrap' and 'Glitchtrap' in the summaries, though their search is less precise. Tumblr remains an underrated hub; following blogs dedicated to FNAF lore or Glitchtrap fanart often leads to reblogs of fic snippets or links to where the full stories are hosted.
Another angle is to look for stories that explore the concept of 'digital consciousness' or 'AI corruption' within the FNAF tag, even if they aren't explicitly marked as a crossover. The premise of William's essence invading or merging with the Glitchtrap anomaly is a natural fit for those themes. Sometimes the best finds come from recommendations in the comments of popular FNAF fics that tread similar psychological ground. It's a bit of a treasure hunt, but seeing how different writers interpret that monstrous fusion—whether it's a battle for dominance inside a system or a collaborative digital nightmare—makes the search worthwhile. I just finished one where Glitchtrap wasn't a separate entity but a corrupted backup of William's mind, slowly overwriting everything it touched.