Yes, you can splice off the power going to your Sony deck. Just need to figure out which wire is 12 vdc and which is ground. For 12 volt
systems, white is typically ground, but I'll bet you'll find more than white wire going into the Sony, since it has speaker connections as well. Look for a ground lug on the Sony - that will give you a clue maybe. A volt meter or test light will be your friend in trying to figure out which wire is 12 volts. Typically red or black, but check. Don't assume anything.