How could I keep the Radio in my truck from losing it's memory?

Alright, I drive a tractor trailer for a freight company. Of course, there are no radios in the truck. What we(drivers) do is build a box with speakers and a car radio mounted in it. There are bannana plugs on the dash to get power from. Now The only problem is that everytime you shut the truck down the radio will lose it's memmory (presets, Volume level, etc). I could get an analog radio but that is just out of the question. I was thinking more along the lines of a secondary power source. Like connecting a 9V battery connector to the memmory wire so when the truck is off it will still hold it's memmory. Seeing that the radio won't work without this wire being hooked up to the trucks power I would have to incorporate a diode of some sort so I don't fry the battery. What kind of diode should I use and Where should I put it in the circuit? I can't mess with the truck, so whatever I do has to be done in my radio box. Thanks for any info....

A 9V battery will hold the memory in your radio. A lot of people use that trick to keep their radio and their on-board car computer from losing its memory when they need to take out the car's battery for repair work. There is even a commercial product which is a 9V battery connected to a plug that fits a cigarette lighter, so its use is well known.

What you need to do is put the snap on connections for a 9V battery into your box across the power input wires (watching the polarity red/black wires). Now you can snap on a 9V battery on to keep the radio powered up. I don't believe you need the diode but it doesn't hurt to add one. Put it in series with one of the leads from the 9V battery so it would oppose the flow of truck power into the 9V battery. The arrow symbol on the diode should point toward the black wire (negative). You can use a low wattage diode since the current required to keep up the memory in the radio is very small.

there is generally a memory wire or automatic antenna wire. it is smaller than the main power wire. generally yellow. if you connect that to a hot spot that always has power like the battery than you will not lose your memory

You can't hook it up to a 9 volt battery because it take 12 volts to hold the memory, the radio is a 12 volt system. Two choices run a wire from the battery of the truck to the yellow wire of the radio that will keep it hot. You could also buy a small motorcycle battery or lawn and garden battery and hook it up in your "box" but it will go dead and will need to be recharged after about 2 weeks or so.