[color=#000000
ost_uid0]18 sounds a bit thicker than you may need and would add some decent weight to the car. would certainly be very solid tho !
for a shelf from scratch, you could happily go with 12mm. But I suppose it depends on the weight of your speakers and if you want to mount anything else to it like an amp or some crossovers on the underside.
btw, mounting amps upside-down is generally not reccomended but many ppl have done it with no troubles :
:
If you are not concerned about the weight of the wood, I reckon look at finding the thickness that makes the shelf fit flush against the back of the chairs and the side walls. so it doesnt appear to be a lump
Dave[/color
ost_uid0]