If it's like a after market one there is a wire that gets grounded when you depress the parking break. So just find that wire and permanently ground it or install you a toggle switch to a ground. Usuly a pink or purple wire. Are you referring to a stock unit or aftermarket?
What kind of radio is it? If it's a newer model, most of them require a double pulse ground AFTER the radio is powered on. Meaning you can't just ground the parking brake wire. Easiest thing to do regardless is use a relay. 85 and 87 goes to ground, 30 goes to the parking green brake wire on the head unit, and 86 goes to your blue/white amp turn on wire on the head unit. So when your radio turns on, about a half second later your amp turn on wire sends current to the relay activating the "ground" to the parking break. Let me know if you have any questions. There's also a good video on YouTube here http://youtu.be/lDGZEZrUdRo