well thats something that really should be tested. and honestly fatmat is cheap for a reason .
but i would start with you roof, not too too much there, your doors, seal them up, maybe a layer or 2 on the floor. i would stay away from you ext cab 1/4 panels and ext cab floor. but do the back wall. how much youput on depends on how much power you run. s10s need some natural flex for them to properly get loud. the less power the less mat. but fatmat is horrible mat and will take more material to get the job done right. i recomment secondskin damplifier pro black. sure it cost a good amount more.but 1 layer of it is probably better then 2 layers of fatmat. IMHO
this is with a lot more power, fatmat (roof) and all electrical upgrades.. Same song. The difference (flex) is easily noticable even though I had all the uopgrades..
Testing before and after ther fatmat with the same power was~1.5 db gain with roof only
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