I can confirm Research Design Specialists IS legitimate - although I've never received an email solicitation. Only ever received via snail mail USPS.
I used to get these on an annual basis when I had my Mazda from 2010-2014. You show up to their office, fill out surveys/answer questions about different vehicle specifications and designs...which one you liked better, what you thought about it, how much more would you pay for a vehicle that goes 50 miles further, etc...and they give you a check at the end of it. The email/mail you get should say how much you'll be compensated and how long it should take. The ones I got were usually for $100-$150 or so and they usually only took about an hour.
It seems like car companies hire these research companies to conduct surveys and use that data to determine how to best build their next vehicle based on what people have said in the surveys. So if I remember correctly, you also have to sign some sort of NDA because you're seeing what could potentially be the next Audi...or the next BMW...or the next Honda etc. etc.