The aftermarket head unit only turns on when the key is turned to start, but not accessory and the car won't save presets or the time, ect. I took it out and found the previous owner wired all four red and yellow wires together (one is constant power for memory and the other is switched power for turning the amp on I think). But when I rewired them red to red and yellow to yellow like it should be, the radio doesn't turn on an all. What is going on here and how do I fix it?

 

It sounds like the wiring coming in had issues so he just hooked it up any way he could make it work. You said you re wired them red to red, yellow to yellow. Does it have a harness that plugs into the factory harness? In aftermarket radios, the yellow is usually the constant for clock, presets, that kind of thing. The red wire turns the radio on and off. They usually have a blue/white stripe wire called remote turn on for an amp. Sometimes if it doesn't have that, you may have to use the power antenna wire. I doubt the factory wiring is red and yellow so that's why I ask if there is an aftermarket wiring harness.