I have a '82 300D sedan, and improved the audio by replacing the head unit with an Alpine, and Helix Esprit speakers (4" front, 5.25" rear). I was able to fit the front speakers under the original grilles but not the rears.

The sound was great, but no bass whatsoever.

My goal was for sound quality, not mirror shaking bass to annoy the neighbors.

I could not find a powered sub that would fit easily under either front seat.

Instead I got a JL Audio CS110LG-TW3 (Thin 10" sealed subwoofer enclosure.) and a 300w amp.

The amp was installed under the rear bench seat, and the subwoofer went into the trunk.

Initially I could not hear the sub, until I took out the first aid kit and tray. Once the tray was out, I had plenty of bass.

I'm still messing around with the subwoofer position, but I'm leaning towards firing the sub upwards towards the first aid kit.

Also, my installer is going to alter the first aid kit tray so that the rear deck looks stock-ish. I'll post photos when it's done.