I just noticed that my front left(driver's side) is not work. Does anybody know the way to check amplifier in door or head unit, what is wrong?

If you pull the interior panel off of the door you should be able to tell if the speaker is dead by moving it in and out (it won't move much and will sound really bad like there are rocks in it.

I had one seize up on me.

If it is electrical you can use a multi-meter to see if it is getting voltage at the speaker.

Yes, it electrical problem I suppose, I have an amplifier from door in office now to write down its ##, speaker it seems to me is well. I need to know what connectors/wires I should check. To specify, what wires which goes from head unit to door amplifier need to check to define faulty part for sure

The pin out is looking at the connector with the release button up.
Going left to right, top first.

First one (w/r) goes to the battery (+)

next 4 are empty

last on the top is positive to the left front tweeter (R)

Starting from the left on the bottom:

Ground (-) to battery B/Y (R)

Signal from the main amplifier (R/W)

3 blank then the last on the bottom is tweeter (-)

The diagram for the front speaker off of the door amplifier shows only number 1 and 2 it doesn't have the wire colors.

So for the front speaker:

The plug that the speaker connects to.

The pin closest to the button to release the connector is negative.

Furthest from the button is positive.

Thank you 4 years to Supercharge!

Hope I have time this evening and check it immediately.