headunit cable

[haitchy1]

im putting my new kenwood headunit under my steering wheel. at the back of the old headunit/ heater box, ther is 3 boxes with wires into them. ino the brown/ middle 1 is the speaker cables, the black/ bottom 1 is the power and the top/ other 1 i dont no. i need to no how to power my new headunit from the old 1. if i connect the power block to the new headuit loom it powers my new headunit but my heater cant be powerd. how could i connect both from the 1 power source. and how could i make it so when i turn my ignition on and off it turns both new headunit and old heater unit on and off.

thanx for the help

ben.