We've already tried email campaigning and cold calling.
If you have a marketing budget (even $5/day will elicit much more response than you'd expect), there's tremendous untapped opportunity for your niche. Based on the description in your question, I think Facebook advertising would be smart, targeted and provide the best ROI.
1. It's simply the most cost-effective way to engage an audience in a specific niche like you're targeting.
2. It works great for B2C and B2B.
3. Unlike Google AdWords and other PPC options out there, Facebook makes it much easier for entrepreneurs and business owners to get started and manage campaigns.
I'd recommend going to https://www.facebook.com/business/learn/ to get the basic knowledge you need to test a campaign. There are literally hundreds of other resources online that go into more detail on creating your business page, choosing audience and objectives, ad layout and other useful tips.
Hope this helps provide some direction :)
That question cannot truly be answered unless one knows the nature of the offering, the level of company targeted, and who you consider to be the decision makers.
Automotive OEM's have very strict processes for procurement. The tier 1 supply base will be similar. Tier 2 and 3 suppliers will be approachable.
That said, if you are targeting OEMs and suppliers, social media isn't going to work great. These companies may use social media to promote vehicle sales, but their decision makers and buyers are not using it to source products and services.
The automotive industry in the US, and particularly the Detroit area, is very close and people have worked together for years. Building relationships within the organizations is your best opportunity for success.
Without knowing what you've sent or done it's hard to know what to advise. If your efforts are all about trying to get in front of someone to say look at my stuff and the and the features it has that's an up hill battle and won't be very effective. There's ways to warm up the leads with social media if they are using it. Look for ways to provide value and build relationships with the decision makers and the gate keepers. A sales coach might be able to help with ways of getting past the gate keeper.
I'm inclined to agree with the other posters here that social media COULD be an avenue to explore. That said- are you in an urban market, or somewhere a bit more rural? The auto industry can encompass a great many things- are you creating and selling new parts, or are you offering a detailing service, or anything on the very wide spectrum in between?
My point is that there are likely quite a few avenues that will be much more productive than email and cold calling. Check for local trade events (Google is a good resource for that). For less money than you're probably spending on emails, you might be able to get a table and display board at a nearby exhibition. A weekend of face-to-face time with decision makers goes a long way. You could have opportunity to get in front of some local media (the news loves 'soft' stories about new businesses in the neighborhood). You might even be able to offer a sample, similar to a portfolio or proof of concept. The key is to find a way to differentiate yourself. You don't need their attention for long, but you do need to get their attention.
Give me a call- I'd love to discuss this with you in more detail.