The relay for the interior heater is the green 'ignition' relay, numbered V on the main fusebox body.
Haynes manual says front fogs are relay C, dont know the colour thou.

I'm guessing there could be a poor connection on one of the connectors causing a heat build-up. it's probably worth a look.

And you could check for any corrosion on the metal bus-bar underneath where it connects to the power wires (red and blue ones). I had corrosion here on my car which stopped the heater working.