I went through a similar situation with my '96 E300D. The problem the dealer had to deal with was not having authority to replace certain parts. The regional rep knows no more about it than the dealer, but they can authorize paying for parts that they can install in an effort to fix it.

In the dealers defense, my problem was intermittent, so they had to drive it a bit before it acted up for them. Having worked on all sorts of electronic and computer equipment over the years, I understand the problem associated in troubleshooting an intermittent situation.

If they have taken the car for a few days, I fully expect that they will find and cure the problem.

Good luck,