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Depends on car. VX Commodore has very large fuse panel under steering column you could tape something to reverse of. Underneath lids of fuse boxes in engine bay if it's not too hot. On the VB, there are panels under the dash that can be made to come away very easily. My VB has no dash speaker, unclip the dash speaker grille for a litre or two of room inside. Hides under dashmat anyway.
I find it disturbing that I thought of these things just now off the top of my head lol....