Display is out on KDCX590. Someone (tech48) said the problem is not head unit but inside main unit and that something could be soldered...Need directions please.

I would need to look at the service manual to be sure this model number is the same problem, but I think it is.

If you take the radio out of the car and open the top on it by removing the gold screw on top near the back middle, the top metal plate will pull off and you will see the cd player.
After that you look into the cd player and it is held in by 2 gold screws. One on each corner in the front of the cd player. After removing those 2 screws you will be able to take out the cd deck. Be careful pulling it out, you can damage the ribbon cable connecting it to the main board. Set it over to the left and look into the unit on the main board at the front right area of the board. If you see a series of surface mount resistors, this is the same fix as the other model. If there are not a series of resistors the problem is something else. Maybe the connector for the display or the lcd itself.
Those surface mount resistors need to be resoldered. If you do not know how to properly resolder them, I advise you to take it to a service center to have it done. If you do know how to resolder surface mount components then go ahead and give it a try. Make sure you don't make any solder bridges that could short out another component.
Those surface mount resistors are heat stressed and need a touch of new solder put on them. You can do it without taking the main board out of the unit. They are on the top side of the board in plain site in the front right corner all lined up one after the other and in one spot two that are side by side.
I have fixed this problem over a hundred times, It applies to several models, but I can't remember off the top of my head if yours is also one of them, but it does ring a bell.
I will check in the service manual tomorrow when I am at work and get back with you if it is something different.