When you say sat nav do you mean you have all the MGR components (head unit, amplifier/tuner, gps box, aerial, etc. etc.)? And does the hu look like this one?
If so then the missing bit is the correct wiring loom for the car and that could be your biggest challenge.
If your car was built without sat nav then you don't have the wiring fitted in the car needed to hook all the components up and get them working. I did read of someone who put a totally new loom into their 75/ZT to sort this but it was not a quick or easy job.
I'm sure others will be along with more advice but you really have picked a big one there. Good luck
Ok. I may be wrong but I expect you have much of the cabling that connects various components but are missing the main car wiring loom that carries the 'core' cabling for the system.
Not sure whether it helkps but the system is a BMW sat nav system and cabling, lifted almost straight from BMW circa 1999. That means you will find a lot about the system on BMW forums.
I don't myself know where you could source the wiring and component diagrams you need-sorry
But I do know that even if you had everything (inc main loom) and knew where it all went, you would need to take the car apart to a fair degree to get it all in, set up and working.
Sorry that I can't be more positive but, due to the system being 'built in' to the car at the factory, and the core cabling not being in all 75's built you might find it easier to follow Rich's advice and get a good after market system.
personally i would flog all your stuff and buy a tom tom go or something. The nav ties in with all the other ecus on the car and is a complete nightmare to try and install and setup as a retrofit. Even if you did get it all fitted adn the wiring complete, it still needs to go onto testbook to be configured/enabled.
Seriously i dont think its worth the hassle as a retrofit.