Depends on - what age your defender is pre / post Td5 & how deep your speakers are.
I got a decent set of speakers in mine placed into the lower section of the dash, but I have a Td5 wiper motor. Pre Td5 the wiper motor is in the part of the dash so you will need skinny speakers.
You could put them in the door, but they will proberly need to be waterproof!
Or in the headlining, there is a company that makes custom headlinings for defenders with speaker pods built in. La Selle or something like that - they advertise in the land rover mags.