Are you sure you even need a car? Where are you staying in Grenada? You can take the bus to Grand E'Tang from St. Georges.
One thing to be aware of if you are driving in the mountains is, as noted, the roads are very narrow. Often times the driver will sound his horn when approaching a blind curve, I once asked why and was told it is so that if someone is coming from the other direction they will know to stop and allow the first vehicle to go.
We've been al over the island by bus, including Spending a day going completely around the island stopping in Sauters, BelMont Estate and Grenville.
Grenada is not like St. Maarten, while I wouldn't leave a purse on the seat or a rolex on the dash, nobody is going to steal your luggage or beach chair from the car.