On the Farm Road track, go under the train trestle in the middle of the pond to find $1500.
On the Restricted Area track, go straight when you pass through gate 5. After a little driving you will reach an aircraft hanger with a jet inside. In the corner of the hanger is a chest of cash.
On the Laguna Del Ray track, off on the right is a small island just offshore. On top the island is a chest of cash.
Between gate 6 and 7 on the Black Gold track is a fenced-in area that contains another box of cash. There is an oil truck nearby it.
When starting on the Treasure Bay track, go over to the beached tanker boat to the right, and drive around on the water side. You can find a chest in the side hull, with $666.
On the Truck Stop 101 track, go to the "hidden" highway and drive alongside the right side of the road. You will eventually find a chest of cash.