It's been five years since I did my TriStar carpet, but as I recall I cut out the front triangle bit from inside the underseat storage compartments where the seam would be hidden. I was able to slide the new piece under the seat base. I may have used the jacks under the dash to help gain a bit of clearance.
2005 X2, Viper Red, MCX, OJ TZ-X5 prop, PPass, rear 750 sacs, KBS, IBS, Bennett Wake Plate, custom wake shaper
(previous) 2001 X5, 1991 TriStar 190