In article ews.net>,
"Joaquín Topiso" > wrote:

> When I am driving for more than 30', the interior part, close to the
> driver's right leg gets hot.

This is more than common--it's an inevitable part of owing an older
MX-5. Under the decorative vinyl outer shift boot, there are two rubber
inner boots: a lower one on the transmission to keep dirt out of the
shift turret, and an upper one to seal the hole in the floorpan where
the shift lever enters. The upper one is torn, allowing hot air around
the exhaust system to enter the interior. Chances are the lower boot is
torn as well, so plan on replacing both of them. Might as well clean and
refill the shift turret while you're in there. See
http://www.miata.net/garage/shiftboot.html

Another cause is an air mix door that doesn't close all the way, so the
heater cannot be shut off completely. To adjust the cable, remove the
glovebox and the rest will be obvious.