Parking can be at a premium in these locations. As far as private spots in hotels, reserve well in advance and bring a small car - spots are tight! I would not keep the car if you're staying in a city.

GPS is very handy as even route atlas don't give enough for towns (we used Windows-based mapping programme on a laptop with GPS receiver for a different reason).

As far as autos go, still very rare unless one rents a big car which are expensive. Try Europcar (generally the best rates in France for small cars at least) or Sixt.

p.s. Marseille and Nice are both in the Provence region. Or do you mean Aix-en-Provence?