you're going to have to change your seals, it's a routine maintenance item, especially on verts, but most people ignore it, over time the rubber seals become brittle and crack, which causes the leaking, I work at a dealership and pretty much every e36 customer car leaks when I take it through the car wash, z3's as well, I'd suggest going to the dealership and having them do it, or if you feel like changing it yourself you can take it there and have a parts guy look at your car so you can determine which seal needs to be replaced
looks like number 3 in that picture might be the culprit, the cowl gasket, i'm not sure, you can study that diagram if you want, check out realoem.com if it's not in that picture
here's a link to that section
RealOEM.COM * BMW E36 325i FOLDING TOP MOUNTING PARTS
p.s. I like the SS in the background =]