 Firstly this is the second SLK I have owned.  The first being a brand new 2014/ (2015) SLK250 model.  The first 6 months it ran like a dream, and then during winter, I noticed an odor occasionally coming into the car.  The closest description that I could offer, that other passengers identified too, was like a burning crayon smell.  I tried to work out when it was occurring, and the possible cause.  After a few months of deliberation, a myself and a few passengers of my vehicle came to the conclusion that the anti-corrosion materials (coating) used in the body of the car, had was melting a low heating point, and at some point hitting hot parts in the engine compartment.  This would start out as a slight odor, but over the coming months intensify as the days got hotter, to a bad odor, and fumes that were now getting into the cab of the car, with the top up, or down.  I reported this to the dealership, and it was a game of tennis over about 4 more months going back and forth with them trying to replicate the problem when they were in possession of the car.  They reported it to the regional technical rep (in NJ not far from MB USA headquarters), and after the first attempt of adding more of the soundproofing materials to the engine compartment bulkhead, and stripping the doors and cleaning them out from a very large amount of the anti corrosion / wax substance, seemed to make the issue worse.  The dealership did was as helpful as they could be, but handed me off to corporate to try to come to a resolution.  The only resolution that could have corrected the car, was to strip it down to the frame/body, and dip it in a bath that would remove all the anticorrosion materials and start again.  Not practical on any car, so I was offered a buy back deal.  After many months of passenger headaches, myself wheezing and breathing issues from the car, a settlement was reached to buy back the car and purchase a 2016 model (SLK 300), which I currently own.  Now my theory was, that a different model, at a completely different time off of the production line should be far enough away from the original car in time etc, that is should not have the same problem.  The old SLK 250 you could see the wax like substance coming out of the door through the rubber grommets and dripping down, also from the engine compartment when you opened the door etc.  Now I have had the new SLK 300 for a period of 5 months when I started to notice the same problem.  Starting with a slight wax/crayon smell, and as the next few month rolled on, and the heat of summer hit on top of the cars engine heat, the exact same issue with fumes coming into the car, when driving, generally any speeds above 40 mph with the top up or down.  With all this, I can only assume it's the entire production line that has this issue, and the routings in the car's assembly instructions and details have had too much of this compound specified so as to prevent rust, but not melt at low temperatures and overspill to the engine compartment and hot parts around the bodywork.  A complete shame, and downside of an otherwise fun, and fantastic car to drive.  True roadster, and enjoyable drive.  With this, happening, and from what I can find using google across the last 5 - 8 years of owners, I am not the only one with this problem.  I can NOT in any way recommend this car to anyone, or say I will every buy  this model of car again, and as it was such a fight to get Mercedes to recognize the problem the first time around, would probably never buy a Mercedes again, even if this issue was limited to this Model of car.  If you find this review helpful, then my job is done.  If you decide to take the gamble, I can only offer you good luck, and hoping your car/model does not have this issue, you have as much fun as I have had driving this car, prior to these issues arising.  GOOD LUCK, and FUN SAFE DRIVING.