Jason, I think you're being put in an awfully unfair position. Make a change without changing anything??? In reading all the above comments it would appear that no one has really addressed your question, so I'll do my best for you.
The only thing you can actually change that has no effect on the budget is the process. I'm not sure if you're talking about internet based email leads only, but lets assume you are. Who currently answers these leads? Who is managing what happens to those leads after they do come in? Do you have a solid follow up process in place for these leads? Just because lead volume is down, doesn't mean that you can't improve your close rate.
What I would tell your general manager is that after evaluating everything, you will not be able to produce MORE leads without altering the spend or getting an increase to the digital spend budget. You then tell him that your plan is to focus on selling more of the leads that do come in, as that is something that you do have control of. From there, show him your current internet lead close rate going back three months. Take whatever that number averages out to and add at least two percent. Ask him to give you the next three months to show him the new process is working and that you were able to deliver the improved close rate. More is not always better after all.
If you're unable to improve the close rate, and you can't get the GM to understand the importance of properly funding the internet department and/or digital advertising budget, I would then look to go to work for a dealer that does. You would be surprised how many dealers have no one looking out for the best interests of their digital advertising investments. If all of this sounds like too much, consider floor sales. Nothing about managing the digital side of the car business is easy.
