Hi Lisa,
Are you asking how to tell how many of leads of a specific source convert to sales? As in:
I got 10 leads from Cars.com, and I want to know how many I sold? If that's the case, then you should be able to do that through your CRM. You just have to make sure that whomever recorded the lead reported the right source. Most CRM's allow you to build reports based on source, and you'll be able to get the data in a snap!
If you are trying to determine the source of the leads on your own website (Google, Cars.com, etc), then you will need to set up goals in your Google Analytics. Dealer.com sites are the easiest to work with on this since you just have to punch in the URL within the Goals you set. Some other websites you have to place a small piece of javascript in the page to be able to track it. Raven Tools has a program that creates the script for you so you would just have to email it to your website provider to install on your VDP's.
This will allow you to see exactly where your leads came from on your website, but won't provide a name to the lead. The only way to get both is by utilizing cookies/session ID's, and working with your website company to set up the reporting. I did this with Autofusion when I worked at a dealership in Montana, but any of the main website companies should be able to do it. This will allow you to see who submitted a lead on your website, where they came from and then you'll be able to track to see if they bought. This is a fantastic resource for banner advertising.