Had same problem with 87 420 SEL for months, then spent $5 on new gas cap, problem solved....folks at European Autoworks say it is a common problem. If cap is not completely sealing 100%, fumes back up into trunk and thence to passenger compartment...I noticed it especially with sunroof open and windows up, which created a slight vacuum in the passenger compartment at speed that would draw fumes from trunk...twice stopped and checked under hood and car for "leak"....new cap fixed it.

JoeB.