You must have a 90 that was produced before 7/90.
There was no wiring harness available for those models (like mine)
Your going to have to do the wiring yourself, without a harness. http://www.bmwe34.net/e34main/Upgrade/Stereo_radio.htm
Check out that link - it should explain a bit - this isnt as big of a task as a lot of ppl think it is.
If you need more help, i should be here to help