I use Airline network/Dollar who are excellent but sat nav is not an option.
Someone told me the other day Hertz offer it so I just checked and compared the price for our Xmas trip.
I paid £220 for full size car 11 nights Platinum Insurance with Airnet
Hertz.com for same size car with satnav ($89.55) added, total $1156 without top up insurance.
Maybe there are some codes out there but at that price I would want about 60% discount. It would be cheaper to take satnav with you as we did last month, Only an ipaq PDA with GPS and TomTom software but it was great.