I went through this on my 93 SL500. it drove me nuts. I walked away for a bit, but do make sure your cap and rotor are clean(do not assume). Use a dremil and sand paper to make sure everything is clean, and that should solve your backfiring...

Some of these cars have switch on pedal, and the switch works the moment you push on the throttle. The switch may short, or not work. My buddy with his 93 W140 had two or three of these replaced at the MB Dealer in the late 90's. The switch on the bottom of the carpet can short closed(sorry I cannot remember the name of it). It trips when you do a wide-open-throttle. Make sure you can manually toggle it, and measure the continuity with a ohm meter...

Martin