The radio relay on a '91 is basically a standard Bosch relay. The only difference is that it has a 10A fuse sticking out the front that's internally wired to the relay output. So probably any fused Bosch relay will work if you swap in a 10A fuse (actually, any unfused Bosch relay should work too, but in that case you should probably wire in a 10A fuse somewhere).
Before you replace it, though, what makes you suspect the relay? Have you troubleshot with a multimeter first? Can you feel the relay click when you turn the key to on? When that happens do you get 12V to the radio or not?