The easiest way is to open the trunk using the emergency trunk latch release. The trunk latch release handle is located on the driver's side sill plate, just behind the driver's seat. When you pull the latch release handle, the trunk will open. Here is a step by step guide:
1. Open the driver's door of your 1991 Mazda Miata.
2. Locate the emergency trunk latch release handle on the driver's side sill plate, just behind the driver's seat.
3. Pull the emergency trunk latch release handle.
4. The trunk will open.
