I'd think its the muffler getting hot and radiating it into the interior.
My floorboard gets pretty warm right behind the passenger front seat right above the muffler and my muffler is 20 years old so it could use a changing.
Have you bedlined the interior? That will help keep heat and noise out.
_________________
'02 Jeep KJ 4x4- Gone
'92 YJ- lift, swampers, bumpers....yeah, its pretty sweet.
*DISCLAIMER*: I can not be held liable for anything I say on the forums or in real life.
I also can not be held liable for any jeep breakage or life problems caused due to my advice being taken.