Officially, it is a 1N4004 diode. Rated at 1 Amp. When installed in this manner in parallel with a load, such as a lamp, it functions as what is known as a 'flyback' diode, which helps to reduce the voltage arc across the contacts of a relay by helping to dissipate the current that will try to flow across the contacts of the relay as they open or close. Suppressing this arc will improve relay contact life.
Pay attention to the white or silver stripe on the replacement diode. In my B7, the band was installed to the rear. Now that I popped open one of my lenses to look at the diode, I cannot get the darn thing to stay on. It came off easy enough, but no matter what, it will not stay locked on the door panel. Probably got to use a smidgen of silicone, but I don't think that should be necessary. Let me know if you can get yours to stay on.