E30, yours is easy

Take out the roof rack mounting grommit, clean it up, then refit it with some sealer round it, simple as that, you dont want to get involve with breaking the trim clips for the headlining
Johnnyboy, not quite so easy

It might be coming in through the pollen filter housing (under the passenger side bottom windscreen trim) people have a habit of leaving the little metal drip shield out and water goes straight into the heater box!!
Other places are, the windscreens were sometimes stuck in with gaps in the sealer, but not very common plus the screens usually would have cracked by now

, but the most common is under the NSF wheel arch liner, there is a multiplug that goes through the inner wheel arch, that lets water in or I have also seen under the bonnet hinge on the passenger side, the sealer gets missed or partially sealed where the inner arch joins the bulkhead allowing the water to flood in between the panels then it comes out where the multiplug fit the arch!!
Hope that give you both something to look at
