I'm no auto electrician, but cars used to be fitted with a componant called a constant voltage stabilizer. It's purpose was to maintain a set voltage(about 10 volts) to the fuel and temp gauges. This prevented erratic readings with changes in battery voltage. Both gauges shared the same unit and I should think an electric speedo would need the same.
They used to be similar to a flasher unit and mounted behind the dash.
If you go to Renault parts dept and ask they will tell you where it is on your car. They shouldn't be all that expensive.