Either of these conditions would point toward engine coolant and a leak in either the heater core or the control valve.
Bmw e30 heater valve leaking.
If the bmw e31 has a weak spot it s the cooling system.
The value and heater core are separate components.
In particular the heater valves get stuck or blocked hoses explode the auxiliary pump seizes up leaks and then the end drops off.
Water entering from under the floor.
This is the cause of 99 of wet passenger footwell issues.
Guts break replace whole valve with another used valve repeat.
I was able to change both on my 325ic.
Why buy a new valve when the brass part does not wear out the kit includes everything but the brass shell.
Make sure that you do not have a rust through area in the floor.
There is a good article on how to replace the the value and heater core.
It s a simple if fiddly fix made a lot easier by removing the glove box.
The heater valve connects the matrix to the stub pipes and its joints and o rings can break down over time.
I have gone through a few heater valves.
Rebuild is a very simple process.
A quick check would be to confirm hot coolant is entering the valve at the upper hose red arrows and into the heater core through the lower hose green arrow.
If the wetness is not from engine coolant we have a few possibilities.
Hot coolant flows from the engine crankcase to the valve.
This time i bought a rebuild kit from blunttech.
The heater valve can also break if an incorrect radiator cap was fitted.
You will need to replace the two o rings that reside between the valve and the core.