Many people think you can wire a tweeter directly to a speaker and that assumption is partially correct. You'll also need a frequency crossover to complete the installation and prevent damage to the tweeter because it's not meant to receive the same amount of power as the speaker or woofer. A crossover adjusts the frequency and power transmitted to prevent speaker damage. The tweeter takes the high frequency sound while the woofer takes the low frequency.
Disconnect the negative terminal from your car battery to prevent short-circuiting any electrical equipment.
Remove the speaker's grill. It also may be necessary to remove panelling to remove the speaker, depending on the make of your vehicle. Unscrew the speaker from its mount using a screw driver and disconnect it from its wiring harness.
Solder the crossover's cable input leads to your stereo's speaker output wires. This will be the speaker wires that were originally connected to your car's stock speakers. Be sure to connect negative leads together and positives leads together.
Connect the crossover's high frequency leads to the tweeter's speaker wire terminals in the back of the tweeter. Be sure to connect positive leads to positive and negative to negative as you did with the crossover.
Connect the crossover's low frequency leads to the speaker's terminals located in the back of the speaker. Once again, be sure the crossover leads connect to the identical terminal.