I have a Defender on order and had the same question. I checked for info on the headliner and everyone said that 6.5 inch worked. Read the Q&A on the product on Amazon.
https://www.amazon.com/Defender-front-liner-overhead-715003098/dp/B01ENM2QI4/ref=sr_1_2?crid=2WCSJVRF4ZTPF&keywords=can+am+defender+headliner&qid=1651169032&sprefix=can+am+defender+head+liner%2Caps%2C103&sr=8-2