Try a different mounting position for the antenna and make sure the base is properly grounded to the body. Putting an earth wire to the battery or any other part of the car wont work.
The main problem with defenders is that all the body panels bolt together and some may not be as well grounded as others due to the panels being painted before being bolted together so you need to mount the antenna to a part of the body that has a good ground to the bulkhead.
Check to make sure the center part of the coax isn't grounded, check this with a multimeter.
Is it 3 right accross all 40 (or 80) channels? If you have an 80 channel rig, check the swr on mid band, if its lower on mids and high on muppet band then the antenna is too long. A lot of antennas on the market today are tuned more for 26mhz than 27mhz. I had to cut 2 1/2 inches off my sirio 4000 to get a 1.3:1 swr right accross the board.
If you find that the antenna is too long even at its lowest point then start cutting it a 1/4 inch at a time. Cut it at the bottom, not the top. Check the swr after each cut on mid and hi bands and make sure all the doors are closed and you're in an open space away from buildings, metal structures etc.