As long as the front engine panel is fully sealed and the carpet is in place, you shouldn't be feeling any heat from the engine bay in the seats. If you place your hand between the seat and the firewall, do you feel heat radiating from the firewall area?
Check the option codes listed on the label under the front trunk lid to see if your car had the heated seats option. If not, maybe remove the drivers seat to make sure that a previous owner didn't install an aftermarket seat heater.
If none of these ideas check out, then you're most likely just feeling the heat of your body being trapped against the leather seat. Leather isn't the most breathable fabric. You could look into options like perforated leather or some other fabric that an upholstery shop could use to replace the center panel of the seat cover.
Beyond that, it might be time to cut back on the burritos.
