I found this spring inside my left door when I put in my new speakers.
The door works perfectly, lock, alarm, window, everything is ok.
What is it?

 

It looks like the spring from the window regulator.

If your window goes up and down okay, it was probably left accidentally by a rushed mechanic.

Marc

 

I was going to say "a pack of cigarettes"!

 

Yes it's a pack of cigarettes! (The pack is there for a size reference, as you might have found out)
Thanks Mark, yes it must be that spring. Does this mean that there is another spring already in place or could things work without it?

 

Which One ?

 

Looks like mosquito repellant to me!