Also, it's more than 10 years since I was there, petit taxis were like taxis in europe but could not leave the municipality.
Grand taxis are/were more like on demand bus routes. There were terminus areas for each route, you waited until all 5 seats were full including luggage, sacks of flour, the odd chicken and the like, before setting off generally sticking to the main road but sometimes going via villages. Along the way people waited at cross-roads until a passing taxi had a spare seat. 'Grand' was because the cars were large ex-european Mercedes and the like.