I have been trying to replace the stock antenna with a shark Finn type for a while now. Read multiple threads. Some says pull it out hard after completely unscrewing, some says to push down and then pull to remove the stock antenna from the socket to bypass anti-theft feature of stock antenna.

 

Tried pulling it out hard to a point where I thought I might break something but the aerial didn't came off. Please suggest a way to pull out the stock antenna and so I can replace it with a shark Finn.

iirc there is a little plastic barb at the end of aerial where it goes into the base. it squeezes in and the aerial slots into base then it springs out inside the base to stop it coming back out. thats the anti theft. you literally need to just pull it Nd will likely break that tab off doing so.

 if your replacing it with a fin though, 

im pretty sure you get the whole thing off inc the base in one go?

pull down the headlining and undo the nut. then 

disconnect the wires from the base.

then attach wires to new fin base,

put in your fin in make sure the seal is a good fit or use some sealant if in doubt and do up the nut nice and tight.