I just did this so here's what I found about mounting it behind the dash. I mounted mine on the firewall right above the tunnel behind the dash. If you stand behind the car about 10' you can see the bottom of it under the dash. So with it mounted there the leads just barely reach the coil which is mounted on the backside of my RF fenderwell. I have also seen the box mounted up on the backside of the firewall behind where the glovebox would be. This may give you a few more inches of lead extention. Recomended if you have your coil mounted where you need more length on the harness. I would start with how much harness lead you need & work back from there.
