Being in the promotional product business, here's my take:
Frames are ok. Walk through a mall parking lot, and I'll bet over half have a dealer frame, 25% have a vanity frame, and the rest have none. If a detailer wanted to buy them from me, I would ask about the demographics of their customers. Are them mostly daily drivers & minivan moms? If so, they might be appreciated / used / effective. Doing mostly exotics? Don't bother-won't be used.
As far as them being on dirty cars. Not a big deal. It's all about name recognition / familiarity, not how something is presented. Think car dealers want someone to remove their decal / frame if a car doesn't look showroom new? Nope. Doesn't matter.
I agree with Scottwax about referrals. It's a personal service business and referrals carry more weight than advertising. The key is making sure your customers are ready to give referrals and have you top on mind. My personal favorites (after a professional appearance) would be cards (something nicer than a business card-laminated or credit card style) for them to keep in thier glove box and hand out. Another cool little item I like is a car coaster. They run from 30 cents (felt) to $3 or $4 (Coasterstone), and are actually useful. Someone also recently asked about windshield shades. Again, I like the idea. Not too expensive, serves a purpose, protects the car and it a great big billboard. One time I did these for a bank on a car loan promotion. You couldn't go through a parking lot in town without seeing one. Made them look like they were doing every car loan in town. Name recognition is key. If you drove through an unfamiliar town, and every house for sale was listed by one realtor (no matter how ugly or unkempt the house), who would you call if you wanted to buy a house?
For promotional product marketing, the key is to think it through in the mind of your customer or potential customer. Don't rely just on what you think.