I have an after market aerial on my car and was going to change it because it's crap

.
I can't get a solid radio signal and the TV has never worked (I know it doesn't matter now as the analogue signal is being turned off but it's the principle).
What is the thing in the picture and where does it attach to on the car?
The coax on the factory aerial is very short and I assume it connects to the box in the pic somewhere in the boot area, where as the aerial fitted now has a long coax that runs towards the front of the car.
Any help appreciated.
Steve