One of my best friends has been doing this for a while on the side and he is doing very well with it.
To become an auto broker all you have to do is contact dealerships and ask them if they use brokers. If they say they do, then ask what their qualifications are, and how much commission the average sale brings in. (my friend averages between $200 and $500 per sale) Now if you want to be an auto broker, there is 2 different ways it works.
Most of the time it works just like referral fees. Basically, you post an ad for the dealership, and use your contact info. You then arrange an appointment for them to meet with a salesman at the dealership, and if a sale is made, you get paid the referral fee.
The second way: Some companies have software that you can load onto your computer, then you actually key in the applicants info and get them approved. Now obviously this way pays more on average than the other way because you are doing all of the work that the salesmen don't want to do.
You can choose to work with just one dealership, or you can work for multiple dealerships. My friend works for only one dealership, and he uses the second method (software), but you can probably make more money if you worked for several companies.
The good thing about this is, that you can do this in any city. Lets say you live in L.A., you can post ads for a dealership in NYC, Miami, Atlanta, Dallas, Chicago........
I know this isn't the normal IM, but it is something you can market online and make a descent income.