Did you double check to make sure it isn't the middle brake light (on the lower part of the rear window)? Many of us forget to check the 3rd brake light.
Also, on the earlier e38s, there is a common problem of corrosion on the brake light sockets. Check yours & maybe use some fine sandpaper if you see corrosion.
2000 740iS. MKIV, 16x9, DDEs controlled by Euro fog light switch, reversed steptronic +/- pattern, paddle shifters, clear corners, Basslink, iPod video with stereo audio, TV tuner, gauge rings, ///M pedals, backup camera with 'on demand' switch, quad brake lights, Dinan engine & tranny software, windows/sunroof close via remote.
2003 540iS. MKIV, 16x9, PDC, CWP, TV tuner, backup camera with 'on demand' switch, paddle shifters, windows/sunroof close via remote, iPod video & audio.