Help- 3rd row seats- 98 ML320

Has anyone added 3rd row seats to a 98ML320?? I removed the plastic caps above the rear wheels to find there are no brackets underneath. I believe the option was available after the 98's... can it be retrofitted??? The
slots appear to be in the right places- and it does 'appear' to be possible... at first glance anyway...
Thank you...

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