Did you tune the antenna with an SWR meter? Tuning the antenna is the most important thing you can do when installing any 2 way radio. I know some antenna manufacturers claim that their antennas are pre-tuned. Than is absolute garbage! That's like saying that your new tires are pre-balanced and don't need to be balanced. The Fire Stick II is a good antenna and it has a tuning tip on top to set the SWR. I just got a 2002 Explorer and am also a ham radio operator. I plan on turning my truck into a mobile comms center. I've been doing radio for almost 40 years and I am licensed by the FCC to work on all mobile emergency communications setups. I also worked for the State PD doing mobile radio setups in the trooper cars.
If you have any questions about any 2 way radio or scanner setup please feel free to contact me. There are some laws in some states that you need to be aware of when setting up mobile comms.
Jaime-KA3NXN