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.
"For every complex problem there is an answer that is clear, simple, and wrong." (Henry Louis Mencken)
2005 X2, Viper Red, MCX, Acme 1285, PPass, rear 750 sacs, KGB, IBS
(previous) 2001 X5, 1991 TriStar 190