i have a 1990 i sold my 95 and took all the sound equipment in it out should i drill a hole through the firewall like how curcuit city did to the 95 to run the main wire to the battery

 

there is a usually a pretty large grommet on the drivers side that you can run wire through. If it is larger then a 4 awg or if you can't fit the wire then drill away but I have not seen many mustangs where I needed to which includes my own 94 gt.