There isn't a lot on the forum so here's my experience. I was getting a P0128 code - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). I ordered a new thermostat from Eurospares. Order a new O-ring (really looks like a round ring with rectangular section) and maybe a new air filter, since you are in there. My car is a 2008 Automatica. It doesn't have a sensor on the body of the thermostat.
There might be other ways but this worked for me: Start by taking off the big engine cover - a bunch of Torx screws and 4 Philips head screws. Next take off 1 bolt from the center of the air filter box, center top area. Unsnap 3 clips, disconnect the sensor plug, and you can pull the top section of the air box out. I left it connected by a small hose with a crimp hose clip, but the whole thing can move out of the way. Then remove the two side inlets - press tabs from the inside so you can get it started, by pressing one closest to the front, then the bottom one, and the oval-shaped intake part starts to push outward. Next pop the top one and the last one at the back will follow - push these ducts outward from center and you'll be able to remove the lower half of the box.
The thermostat has 3 Socket Head Cap Screws holding it - 5mm drive. They are kind of hard to get to without removing more stuff, but it can be done. Loosen the hose clamp with Philips or 6mm hex drive, and the 'stat can come out. Work it toward the front, under the main brace. Work the hose off by careful prying with a thin screwdriver. And some grip-and-pull.
The new one goes in easily. Tighten and put everything back. Lower half of the air box has two lower plastic pins that fit into holes. The side intakes plug back into the air box and snap in place, they are connected to flexible air guides. Pictures show the 3 screws. I used a flex socket to get to the one in back. This didn't require removing anything from below or access from the bottom, nor the side wheel wells. Toughest part is getting a good angle on the 5mm hex screws to crack them loose.
-2008 Quattroporte Executive GT
-Still driving a 1991 BRG Miata