Is it permissible to establish a car park on private land and rent it out for a fee to patrons of a restaurant that sells alcohol, or to the restaurant itself?

If the restaurant is not specifically dedicated to serving alcoholic beverages, and it offers halal food alongside them, then it is not forbidden to rent out the land as a parking lot for the restaurant's customers. This is not considered assisting in a forbidden act. If the assistance is indirect and unintended, then it is permissible.