The little video which
@jamesmartin has posted clearly shows the shape of the removal keys and how they work.
However, at the back of the radio there's a shock absorbing device consisting of a long(ish) pin on the back of the radio case and a rubber bush in the back of the mounting bracket. Over time the pin can stick into the rubber bush which requires a fair amount of pulling force to get the radio out of its slot.
The radio does not just "pop out", the trick here is to get the keys to lock in place and instead of pulling on the keys which can be difficult to hold tightly enough you're going to have to stick your fingers into the tape slot and give it a good tug in order to get enough purchase to get the unit out of its carrier.