For any other car you can find appropriate length (délka) and strength (síla) of spring here: universal parts/universal parts/gas spring of rear door

 

 

 

 

Howd you get a socket on the strut mount bolt with the ball joint through the bracket? (Or vise versa if you installed the ball joint to the bracket after mounting it).

 

 

 

Chinese seller stole my pictures

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I unscrewed the bolt, put the holder on, screw the bolt back. Then put the ball joint into the hole and tighten the nut. Simple.
And the holder can be a little bit longer to make more space.

 

I see now that the nut is on the inside of the bracket, and the ball joint screws into that. Originally I thought the nut in the photo was the head of the bolt that went through the bracket, and the ball joint screwed onto the male bolt threads. This is a nice mod, the stock hood holding rod is a PITA.