[quote]Hello i want to connect two power supplies to a car stereo like in the picture, but i dont know what kind of diodes should i use?[/quote]
[b]Hello ramziukas,
No diodes are necessary in that configuration, unless you're worried about accidentally reversing the polarity when connecting the power of each.
I too am working on a in home car stereo project using a PC power supply and a plug pack for memory backup. Diodes are definitely not needed.
Regards,
Relayer
P.S. The picture you've provided isn't quite correct. The memory backup shouldn't be connected to the PC's PSU (Only the ground wire).
If you were to have your stereo switched off prior to, with the volume up high and you neglect to turn on the PC PSU first when you want to listen to your unit, you could overload the plug pack memory backup.
Please see uploaded image below:
http://i.imgur.com/N3tNzLa.jpg[/b]