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Alright, I got the belt on correctly. The only problem is now that the new belt is on it's way to loose.>>

First, make sure you have the correct belt. They make 4 different lengths for 104 engined chassis and it is quite common to have the wrong length belt .
They come 2415/2440/2445/2475mm. You do not mention chassis #, but assuming it is a '93/300se/140.032, you need the 2440mm.
The adjuster linkrod can be seen between the PS res cap and the thermo housing. If it is there , it is the early style with the belt tension shock just below the Cam Position sensor. It is adjusted until the index marks line up with the pointer on the tensioner.
If it is the newer stlye , there is no adjusters/link rod/or shock.