To remove the thermostat in a 1999 Nissan Frontier, follow these steps:

1. Ensure your engine is cool before starting any work to avoid burns.
2. Locate the thermostat housing. It is typically located on the engine block, near the upper radiator hose.
3. Drain the coolant from the radiator to prevent spills. Place a container under the radiator drain plug, open it, and allow the coolant to drain.
4. Once the coolant has drained, use a wrench to loosen the hose clamp that connects the upper radiator hose to the thermostat housing. Slide the clamp away from the housing.
5. Disconnect the upper radiator hose from the thermostat housing by carefully pulling it off. Be cautious, as there may be residual coolant that could spill.
6. Next, use a socket or wrench to remove the bolts or nuts that secure the thermostat housing to the engine block. Typically, there are two or three bolts.
7. With the bolts removed, carefully lift the thermostat housing away from the engine block. The thermostat should be exposed.
8. Take note of the orientation of the thermostat within the housing before removal. It is essential to install the new thermostat correctly.
9. Gently pull out the old thermostat from the housing. It may be necessary to pry it loose using a screwdriver.
10. Clean any old gasket material or debris from the thermostat housing and the engine block surface using a gasket scraper or a clean, lint-free cloth.
11. Install the new thermostat into the housing, ensuring it is in the correct orientation. The end with the spring should face into the engine block.
12. Apply a thin layer of gasket sealant, if necessary, to the mating surface of the housing or the engine block.
13. Position the thermostat housing back onto the engine block, aligning the bolt holes.
14. Secure the housing in place by tightening the bolts or nuts to the manufacturer's recommended torque specifications. Be careful not to overtighten, as it may damage the housing or the engine block.
15. Reconnect the upper radiator hose to the thermostat housing, ensuring it is securely in place. Slide the hose clamp back into position and tighten it.
16. Refill the radiator with the appropriate coolant mixture, following the manufacturer's instructions. Be sure the radiator cap is tightly closed.
17. Start the engine and allow it to reach operating temperature. Keep an eye on the temperature gauge to ensure it remains within a normal range.
18. Check for any leaks around the thermostat housing and upper radiator hose connections. If you notice any, tighten the clamps further or replace them if necessary.
19. Dispose of any coolant properly, following local regulations.

It's important to note that these steps are general guidelines, and specific variations may exist depending on your Nissan Frontier's engine configuration. Consult your vehicle's service manual for detailed instructions and torque specifications.