If you accidentally connect the battery terminals incorrectly in your 1999 Nissan Frontier, you may experience electrical issues or damage to various components of the vehicle's electrical system. The battery has two terminals: positive (+) and negative (-). The positive terminal is usually labeled with a (+) sign or marked as "POS," while the negative terminal is labeled with a (-) sign or marked as "NEG."

When the battery terminals are reversed, a phenomenon known as "reverse polarity" occurs. This means that electricity flows in the wrong direction through the electrical system. Some common problems that can arise from this situation include:

1. Blown Fuses: Many vehicles have multiple fuses that protect various electrical circuits. Reversing the battery terminals can cause these fuses to blow, resulting in the loss of power to specific systems. To check if you have blown any fuses, you need to inspect the fuse box located under the hood or inside the vehicle cabin.

2. Damaged Electronics: Modern vehicles rely heavily on electronic components, such as the engine control module (ECM), body control module (BCM), and various sensors. Reversing the battery terminals can cause damage to these sensitive electronic components due to the sudden flow of electricity in the wrong direction. This may result in malfunctions or even a complete failure of certain systems.

3. Sparking or Arcing: When connecting the battery terminals, the flow of electricity can sometimes cause sparking or arcing. This can create a risk of electrical fires or physical injury, especially if there are combustible materials nearby. Take caution and ensure that there are no flammable objects or substances present when connecting the battery.

To mitigate any potential damage caused by reversing the battery terminals, it is important to disconnect the battery immediately and correctly reconnect it with the proper polarity. Before doing so, visually inspect the fuses to identify any blown ones and replace them as needed. However, it's worth noting that depending on the severity of the damage, you may need to consult a professional mechanic or automotive electrician to diagnose and repair any specific issues arising from the reversed battery terminals.