try parking differently when it rains (or use a hose) & see what happens. when i bought my mkiv it was also from a previous owner who also was not the best... i would have water pool at first just in the passenger footwell then it got worse to where it would pool in the pass footwell, backseat area floor, and the spare tire well. (yeah, it got REALLY bad) People kept suggesting everything & nothing fixed it, finally got fed up tore down the headliner & replaced / cut all the moonroof drain hoses. Realized the seal on the sunroof didn't seal right from previous collision damage that wasn't repaired right, the body wouldn't conform to the seal properly/completely. Anyway, when i would park with the right front of the vehicle tilted up, it wouldn't leak. At any other angle, it would. Park at different angles, see what happens.