I'll start first with your question about the discount with a Costco credit card. I'm
assuming by a Costco Credit Card, you mean the American Express Costco Cash Rebate Card. Discount car rental benefits are available for participating countries.
For example: you can receive car rental discounts with (links open in a new window)
at participating locations in the United States and Thrifty and Budget in Canada.
Unfortunately, there was no car rental discount with the Costco credit card available for Munich.
There were only 5 travel related discount offers for Germany associated with a Costco Credit Card:
Note: discount with a Costco credit card and Costco credit card offers above, available at the time this question was answered... offers subject to change without notice.
Regarding, the navigation system I checked Alamo, Budget, Dollar, Enterprise, National -- none of these websites offered a choice in their optional items that included a gps. They are available at:Avis , Hertz and Thrifty - you can check a reservation scenario in order to determine the pricing for each company.
Pages from this site, relating to this topic:
Car Rental Discount Codes
International Car Rentals