The heater blower is under the plastic 'scuttle covers' under the windscreen. Open the bonnet, remove these 2 covers and you'll be able to see the black plastic blower unit.
This has quite a large plug connecting it, take the plug off and check the condition of the contacts, chances are one is rusty.
In which case scrape it clean
EDIT: actually that's for the Renault 19, but the clio may be similar