Hi folks, recently bought a 1986 polo and when cleaning the interior, I found a damp patch in the passenger footwell, it's definitely not coming from the doors/sunroof but other than that I can't figure it out

Thanks in advance 

doors are VERY common , i would suggest you check again carefully 

 

if not that then rotten under the windscreen rubber would be my next guess

Drain channels in bulkhead to the wings clear of dead leaves and dirt?

Would have to be quite high water level to come in through fuse box mount hole though.

 

The battery tray side usually rusts through (driver side on RHD car), which I assume is sped up by small amounts of acid from battery gas vent holes. But I don't see why the other side would not corrode too if neglected for long enough.

 

Is it sticky or sweet smelling and is coolant level dropping?

Heater matrix.

 

I found the membrane behind the door card may not be sealed all the way around. Pretty straightforward repair with strong compound.

Or gaffa tape and a rubble sack if the membrane can't be fixed.

 

Is it a factory (sliding metal) or aftermarket (pop-up) sunroof?

 

The factory ones have four drain holes which go down to the back of the front arches and the back corners of the car. It's worth rodding them with a bit of stiff wire. 

 

The aftermarket ones, just, urgh. 

Not sure if its identical in the Mk2 but in the Mk2f the bonnet release cable grommet that seals its passage into the engine bay from the passenger footwell can dribble if its old - mine did heaps when I was parked on a slant that kept a little pool of water around it in the scuttle. Tends to sneakily drain down behind the carpet.

 

Now its sunny wait till its dried out, remove the passenger footwell carpet and pour some water over different bits of the car and watch for the leak. Safe money is on the door membrane though