I made the mistake of buying a S-type without navigation, Now it's my biggest regret. Does anyone have or know where I can buy at least the head unit, besides ebay.

 

It is alot more than just putting in the head unit. The whole body harness of the car is different. Plus you need the module in the trunk. The fiber optic harness etc.
Cheaper to get a nice aftermarket audio system with nav built in and a custom surround made.