Eek, I wouldn't attach it to the head rests. They aren't designed for that, and who knows what might happen in a crash.
If you have just one car seat (NOT booster seat) I would put it in the center and attach it with the lap belt. That belt works better with car seats and your kid will be also be well-protected from side impact intrusions.
I think he'd be very safe there in your 500E compared to a lot of other cars with an attachment point. Assuming you're not testing the limits of that power/performance at the same time, of course, hehe.
By the way, keep your car safe too... get a mat to go under the car seat, especially if it is one with metal bars underneath. Otherwise it will dent your leather and/or lift the finish. Not to mention the inevitable gooey spill. Hmm... now that I think about it, if it was my 500E I'd put a tarp over the entire back seat!
The Mighty-Tite Super Mat is nice... I bought one mail order a couple years back for something like $20 I think.
Mighty-Tite Stuff