Like a lot of cars with a small aerial, mine has a signal booster in the cable, powered when the radio is switched on.

Does anyone know how to test this? I need to understand whether I need a new one, or the reception in some areas is just poor.

couldnt you check it with a voltmeter?

on a side note where is it wired into? the booster i mean, thinking about getting a shark fin, but have no idea where to wire the booser into

The radio has an output to power an electric aerial, usually a blue wire. This is given 12v when the radio is on. Electric aerials are rare these days, but the current use is to power the signal booster, and sometimes the sub-woofer, or at least to the sub-woofer relay. So that's your power source for the sharkfin.

I can test the power feed to the booster, which is ok, and i can ensure the earth is good, but how do I test whether the booster is actually boosting the signal? I suppose disconnect and see if the situation is even worse, but I was hoping there might be a better way.

How do you ensure the earth is good? I have a sneaky suspicion my earth is not that great seeing as my DAB signal drops out randomly sometimes

I've not dropped the rooflining to check yet, was hoping for a definitive answer first, but I believe there should be a separate earth wire and connector. It should be clean, corrosion-free and make contact with bare metal of the car. Its a frightening thought, to create a bare patch inside the roof, so it needs some contact grease to protect it and prevent a big rusty hole.

The earth cable may wander off elsewhere to connect to the car, in which case we need to follow it, often difficult, and make sure its connection at the other end is good, as above. Where earths join to screwed posts, we just need the thread to be a clean contact.