[pimpmcdaddies]

Sounds like a familiar story...Hate to be the bringer of doom but your leak symptoms is the same as mine.I wondered/hoped it was a blocked or split sunroof pipe when I noticed a damp carpet, but after pulling up the underlay matting high in the corners found the dreaded rot.
The problem starts under the front wheelarches where the plastic splash guard clips around the inner wing/engine compartment panel join.20 or so years of road debris collects here which results in rust holes and water entering the passenger compartment.As if this is not tragic enough the water collects under the sound deadeners on the floorpans which then leads to corrosion where the end of the torsion bars fix to body which is obviously not good .
The good news is that the whole front end is held together by 10 mm bolts and comes apart real easy though be a good idea to jetwash under arches first and lube all fixings in advance.
I've ground/cut all the rotton bits out,treated the bare metal ready for welding,welded up and then some good protection.I was unlucky with my Si,it was driven up a muddy track 6 days a week since 1994 but was still sound enough for a good repair.
I also found corrosion at front of inner wings where the air intake runs so 2 more plates required there.I've also put plates above torsion bars and sealed from below.Carpet really needs to come out for this but at least you can check/protect the rest of the floor.
Go on,get it sorted and see if you can get 300k out of the old girl,you'll miss it when it's gone!
Re.Sunroof gasket.I've heard if no new gasket available you can inject the hollow rubber with silicone which helps to make seal with the glass.sounds messy so cover your seats...