Thought I’d taken some photos, but it started to rain.
Basically I had the back seat out, so just went round tapping the metal with my fingertips. If it sounded like a baking tray, I cut out a piece of Dynamat and stuck it on to reduce the resonance. It’s improved road noise quite a bit for me, and the speakers sound great now.
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The front doors are really worthwhile doing IMO, with the speaker out, tapping the door skin behind resonates through the whole thing. Just a couple of squares behind the speaker dampens it nicely.
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You could do a lot more, but I was focussing on dampening resonance rather than a futile task of soundproofing the whole thing.