The exhaust line passes through the tunnel between the seats? It can be sympleously that, that is not well isolated thermally and upload the heat of the escape.
Moreover, it is possible that the catalyst is half clogged or beaten and hot much more than normal.
In any case, there is only one way to check what is heated and why, you make a more or less long journey and when you start noticing the heat in the tunnel, you return home or where you can lift the car and get underneath.
It goes without saying that you are careful what you do that is underneath is you (secure the cat or blur, firm and horizontal soil and others). Once you are underneath, the heat that radiates the pieces will give you what is happening. Anyway, the hand to the gearbox closer, if you do not notice heat, it is the exhaust line and especially the catalyst that is radiating the heat.
All the best.