you posted the question in the 3 series forum, but I assume it is for your E34 520.
There is actually a special tool that is similar to a torx or phillips bit that has 5 sides. You can use a phillips bit, but be very careful not to let it slip.
DIY: BMW Radio Removal - YouTube
On the BMW radio each side will have tab doors with a small flat head open the doors, you will need a allen key or BMW "installer" tool the bmw installer can be purchased from dealer, other wise you can use a allen which will not fit perfectly so you will need to turn it counter clock wise at a angle with pressure to unscrew radio. After removing radio pull up on the harness plug slider and remove antenna wire, This removes your radio for 84-99 3 series, 89-95 5 series, 88-01 7 series cars.
five sided hex key stock radio removal tool
http://i573.photobucket.com/albums/s...w/DSC01683.jpg
http://www.bimmernut.com/forum/5-ser...o-removal.html
http://www.bimmernut.com/forum/5-ser...g-removal.html