Klipspringer: Well assuming the car is simular to this one (Maybe the op can post some photos and tell us which exact model he has?), an aftermarket radio is most certainly an option.
This one is currently on TradeMe, installed with a NZ aftermarket radio.
Certainly, as per my previous post if the op has the double din model pictured above, you can very easily add a new unit, although it will not require any kit such as the one linked to on ebay. As the OP has stated he has internavi then I expect his unit will actually look like this :
Regardless there's no reason to replace either imo as the factory units are great systems.
To the OP I also suggest you check under the passengers seat to see if it has an amp mounted there as there was a factory option premium sound system.
