The OP asked for an easy way out. If he doesn’t mind tuning the radio manually, and doesn’t mind not having a clock, then a switch in the power lead will keep the radio off. Since his vehicle is 12 years old, this would be an inexpensive way to solve the problem. I didn’t say that it is the best way.
You are correct if he wants to keep the radio presets and the radio has a clock which he uses. I am so used to looking at my watch that I couldn’t tell you if my car clock is set to the right time or not.