The easiest one is to put a giveaway bowl in the service waiting area (giveaway a Starbucks gift certificate) - make some fill out cards for people to fill out and drop in the bowl. Make the lines for filling in: Name, Cell Phone, Best Email Address.
Another way to do it is to create fill out cards on recycled paper (maybe put a picture of some grass or a tree) and tell people you're doing your part to save the planet by not sending anymore mail. Tell them you'll be sending out special offers by email and if they'd fill in the card you'll start emailing them instead of clogging their mailbox.
You could even make it a poster that looks like it is very "earthy" to hang over the service writers' heads stating "We want to help save the planet, but we need your email address to do that! ....please give your best email address to your <insert dealership name> service representative."
There are lots of other ways to do it, but these are the simplest.