The most common mistakes made when setting up a Bluetooth headset in a car are those of compatibility and failing to set up the car kit properly to receive the signal from the headset. If the two are incompatible, the headset won't sync with the signal, even if the signal from the car is detected. If the kit is not prepared properly to receive the signal from the headset, it does not detect the car at all.
Compatibility charts are likely included in the user manuals for both the headset and the car. Check the list prior to purchasing the headset to ensure that the devices work well together. Updated lists typically can be found online at the car's accessory site as well.
Preparing the car's Bluetooth system for syncing with the headset is a matter of enabling the car's Bluetooth connectivity and noting the name of the system for detection. Differing cars have various ways of enabling the system, but the specific process for each model should be contained in the car's user manual.
Once the car's Bluetooth connectivity is enabled, turn on the headset and search for the name of the Bluetooth system on the connection list. Proximity is important, as Bluetooth is a short-range signal, and another common mistake is to try to make the connection outside the reach of the car's Bluetooth signal. In such a case, no signal is detected by the headphones. Selecting the car's signal and pressing the sync control on the headset connects the two devices and makes the headset usable.