I want to put a power switch to my car tuner and amp that will allow the unit to be on when the car is off. The current setup uses a hot lead from the fuse panel that taps to the key switch power feed. When my added switch is on it throws power back to the key switch thus turning on the entire dash. I know I need a diode but how to determine the size. 12v with 10 amp fuse. Thanks!
A diode will be possible, but it would have to be quite a big one and it will also drop 0.6v across it, so isn't really an ideal fix.
Can’t you just use a SPDT switch, so that your permanent supply and the keyed supply are isolated?
Connect one of the switch connections on the left to your keyed supply, the other switch connection on the left to the permanent supply and the switch connection on the right to the car tuner.
As @SolarMike says in the comments, make sure both supply lines are fused.
Switch image copied from https://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/3wayswitchR.svg/190px-3wayswitchR.svg.png