If the difference is $4-6k, I'd consider downsizing. It really depends on your goals, and if you are planning to FIRE.
Top Tier - Keep the Benz, and just suffer the payments for 5 years. Your FIRE date remains the same as now.
Mid Tier - You could snag up a brand new economy Kia or something for ~$23k after rolling in the upside down of the Benz. That is if you're really only $4-6k upside. This would lower your debt by $13k. If you invest that (estimated difference) $2600/yr, for 5 years and gain 7% returns, you have ~$16k invested due to the change.
Mustachian Tier - Buy a used Civic with 100k miles for $6k, roll in the $4-6k upside down and have ~$12k debt. Saving $24k. Investing $5,800/yr for 5 years (estimated difference) would net you ~$36k at the end of 5 years if you get 7% gains. Of course, this is assuming the Civic doesn't have any issues causing a replacement. But even if it does, you're only looking at another $6k, meaning you're still way ahead.
So, if someone was willing to pay your husband $36k if he drove a Civic to work every day instead of the Benz, would he do it?
Edited to change math. I had it the calculator starting with an initial investment. These numbers will be more realistic.
**Disclaimer** These numbers are making a lot of assumptions. If any more experienced Mustachians want to correct any errors, please do so.