Subs play immediatly after car turn off

The amp's input is totally dependent on the signal supplied to the rear speakers, so the head unit (the original OEM, I take it?) is the source of the signal surge....and the high level input of the amp adds to the problem because it isn't able to suppress it.  (And I'm going to presume that's what you're using.)

Switch to a high quality aftermarket line output converter, such as one of these adapters here.  The circuitry contained within the adapter should stop that surge when you power off the deck (it goes off when the car goes off).

Also, it's fine to get the turn-on power from a switched source.  It's a good idea also to put a small inline fuse holder and 1/2 amp fuse near where you have it soldered to the car's wiring.

If your gain controls are set too high, this can happen.  This would be the first thing to check.

If that checks out, then many times all it takes is a slight delay to power down the amp before the radio.  What you can do is cut your radio's accessory lead, and install a diode inline (put the stripe side toward the radio).  Now, install an electrolytic capacitor (220-1000 microfarad) on this line after the diode (on the radio side).  The bigger the value, the longer the delay.  Make sure to observe polarity on the cap (it'll pop if you get it backwards)- there'll probably be an arrow with a "minus" sign in it identifying the negative terminal.  Connect negative terminal to ground, and positive to the accessory lead.

Now, when you cut off the ignition, the diode will keep voltage from leaking back into the accessory circuit where the amp is tied on, and the capacitor will discharge into the radio, keeping it on until after the amp shuts off.