It depends on what the voltage of your truck is.
If the truck was converted to 12 volts then you connect the Red accessory wire to the ignition switch coil wire terminal.
When you turn the key ON, that will internally energize the Yellow wire for main system power during standby and full On.
The Yellow always on' wire is connected to the hot side of the ignition switch terminal or any 12 volt voltage terminal that stays 12 volts all the time.
The Yellow wire is for main system power during standby and full On and to maintain clock time and radio station presets.
You could use a toggle switch on the Red accessory wire rather than going through the ignition switch.
If you wired it that way, you would be able to turn on the radio without also energizing the truck's ignition system.
If these trucks came with a accessory position on the ignition switch, there would be no problem.
If you do not remove 12 volts from the RED accessory wire the Head Unit remains ON or in a standby mode which will result in the battery to drain.
For more insight and if you have a 6 volt truck read this
Install Tips 12 volt CD/Radio in a 6 volt vehicle with diagrams attached.
