Joe,
One of the biggest fights of anyone working for an Internet department (in any capacity) is your own showroom floor. Salespeople and Salesmanagers all mean well, but when a customer is in front of them they lose their minds on process....always opting for the path of least resistance. On top of that they're profession is selling people on things, and they're very good at selling coworkers on why the path of least resistance is always the way to go. The truth of the matter is you have to force salespeople to do something - either in pay or through a CRM process. If you're dealing with a group of stores, you really need to do both.
Design your ad source portions of the CRM tool to make each ad source just as easy to select as another. Don't have subsets like Internet - then - Edmunds, KBB, Dealer Website, etc. Have a list like TV Commercial, KBB, Edmunds, Previous Customer, Dealer Website, Radio Ad, Newspaper Ad, etc. Mix it up. Put the least likely things a salesperson would choose at the top and the ones they always choose, like "Drive by" all the way at the bottom. If your CRM tool is worth it's salt, they should have people in support who can help you with this.
You can also give surveys to customers to fill out in the finance office, or other slower points in the sales process, to see in their own words what really brought them to you. Again, you need to be careful how you position/frame your questions.
The positive approach is to constantly (every other day) show salespeople how collecting this data puts money in their pockets. This is tougher to do. If you don't do it every other day, they forget.
On a side note, I know this post sounds anti-salesperson, but I speak from experience. I sold cars for years and am guilty of every salesperson sin in the book. I chose the path of least resistance, I marked over 80% of my customers as "drive-by's" (CRM's were just starting at the end of my selling days though), I believed every word a customer told me, I made deals with customers before talking to my salesmanager....then put my manager together - that was me. I know how to motivate me, so that is where I'm coming from in this post.