I just bought an avic-z2 and I took it to best buy, and they said there isn't a installation kit for the 08 mariner because the dash is different from th 07. The only dash kits I can find are for Mariners that already have nav systems installed, but I don't. What should I do??

I'm not sure on the Mariner, but you may need to buy the factory navigation radio bezel and then use the kit for replacing the factory nav. This is what is required on the Dodge RAM and Charger. Check with Mercury parts to see if the bezel is avalible.

Look for a metra 99-5814 dash kit. It includes a new bezel and all the hardware you will need to install just about any aftermarket radio. It has been my experiance, though, that dash kits like this are not the normal $20 variaty, so most any shop may have to order it.