JPGMC1500

I recently purchased a 2020 GMC Sierra 1500. It did not come with rear fender liners. I purchased the OEM rear fender liners and hardware for $150 from the dealer. Went to install them at my house and instructions say to remove rear fender flares prior to install. Anyone run into this before and had to remove them or got any ways around this? I attempted to install without removing rear fender flares but they didn't fit very snug. They use a lot of salt and brine on the roads where i live so fitting snug is a must.