It's an old car. Old cars rattle, especially with lots of bass. My car ('84 190D) rattles with my puny 8' bazooka. I would recommend dynamat. It's not cheap stuff though. But, rattling like that is typical of cars this age.
I just put 2 12' Sony Xplodes in a bandpass box in my '87 560 SEC. I too have the rattle... Go with the Dynamat! Like he said, the stuff ain't cheap, but it's well worth it. It cost $199 to do my entire trunk including rear wheel wells... I have my box mounted up againest the rear seat with the ports firing towards the back of the trunk. I opted NOT to cut through the firewall behind the back seat & instead cut the bottoms from the first aid kit space & the storage space beside it. Lot easier to get my hands on a new first aid & storage space than to get a new firewall put in... Hope this helps.
To put bass in the 126 body without rattle, do not use a subwoofer that fires into the trunk! Use a bandpass enclosure that has a partially external, square port that exits from the top of the box, through the package shelf. Using a router, round over each side of the port to create a flared opening. Now the bass will be delivered INSIDE your car, not inside the trunk! Have the car stereo dealer carpet the box with the same carpet you have in your trunk. Use a single 12' woofer. Design the enclosure based on the Thiel Small Parameters that come with whatever woofer you buy so it is wider, not deeper. Fit it between the trunk lid arms and when the trunk is open, noone will see a woofer. Show this message to the dealer and he will know exactly what to do for you. P.
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