radio dimmer hook up

my head unit has a wire to hook up to the dimmer switch I installed the unit but never found a corresponding wire in the gaping hole that my stock tape/radio deck left, I am assuming my old radio did not dim therefore no dimmer hookup, my radio is real bright and I like my night time dash dark sooooo....

so my question is how can I run a line from my dimmer switch on my dash to the radio?

head unit is Pioneer DEH-P4600MP
4 speaker non infinity
xj sport 99

you have to find out which is the positive coming off the dimmer (which is basially a rheostat btw) tap into it and run it over to the "gaping hole in your dash" and connect it. Should work.