Head onto the main strip on the west island - the dual carriageway running north/south. Take the northernmost turning into Little Haiti - just south of a building with a green base.
Head west along this road until you reach some sandy coloured buildings on your right and drive into a space there made by the buildings. Now turn around and face south.
You'll see an alleyway opposite with a wooden ramp in it. Drive for the ramp as fast as you can (err... a little slower than that in a PCJ, or you'll overshoot). You'll get a Unique Jump AND land next to the point where the Sparrow ought to be parked (it's not always there - but if you come in by road like this it ought to be).