LDR to 7 segment common anode
[![enter image description here][2]][2] Many years ago I build a circuit to do just what your doing. It controlled the brightness of a LED digital dash I built for my 37 Chevy. After looking at the schematic it looks as though 4 PNP transistor have their emitters tied to the 12 volt source. SK4 to SK10
 I used a photo opto device with buffer to drive a 2n3055 transistor base. 12 volts on the collector and the emitter as the source.
I had to install a very large capacitor at the emitter of the 2n3055 to keep the brightness constant. flickering(1000uf to 5000uf capacitor) Since I no longer have exact valves you will have to design this yourself. The 4 PNP transistor emitters will have to be removed directly from the 12V source and attached to the new emitter source. Worked very well. Bright during the day auto dimming at night.
 In order to get maximum 12 volts from the 2n3055 emitter. I had to over drive the base through a series resistor, with near as possible 12 volts on the base.
2n3055 15 A, 60 V NPN Bipolar Power Transistor TO3 Case.