How do you program a remote for an existing car alarm?

To program a remote for an existing car alarm, first sit in the car with all doors closed. Turn the key to the "On" position (without starting the engine) and then back to "Off" several times—usually 3 to 5 times quickly—until you hear a click or see the locks cycle, which means you're in programming mode. Press a button on the remote within a few seconds to sync it. The locks may cycle again to confirm. Turn the key to "Off" to exit programming mode. Note that steps can vary by car and alarm system, so it's best to check your owner's manual or look up the instructions specific to your model.