Detecting the position of a leak can be problematic and time consuming.
What Leaks? Apart from the obvious king of leaks (the Roadster) it has been known for the Fortwo to let a bit of water in through a few places. The problem is that the water doesn't hang around the area it comes in, gravity helps it move down and usually into your carpets without you knowing. The first thing you usually know is a bad smell or soggy carpets when you lift them up. The Roadster leaks are too plentiful to mention here but the Fortwo tends to be limited to:
Windscreen Door seals Rear side plastic windows Rear hatch.
The windscreen leak is becoming ever more common due to the number of replaced windscreens now being fitted. I'm not sure what makes the smart windscreen harder to fit but it must be, I have heard and been contacted by countless people with a leaking windscreen.
Lift up your carpets in both footwells and feel the foam backing for damp.
Do the same for the boot carpet that covers the engine bay, also feel down the back.
If it is dry then hoorah! If it is wet then you have a Roadster,
a Fortwo with a replacement windscreen or an unusual leaking Fortwo.
Now, the chance of being in the right place at the right time to catch that drip appearing from the problem area and disappearing into the carpet is pretty shallow. Even less likely if the windscreen is leaking because it is only about 2 inches of metal before the drip can hide behind the dashboard.
Pin Pointing Leaks
What you will need is stuff that you probably already have in your house.
Standard masking tape and a NON permanent felt tipped pen.
Apply a single long strip underneath the windscreen just above the dashboard (on the inside).
Do the same underneath each door, under each rear side window and along the boot hatch.
If you believe that water is getting in somewhere else, place a line of tape below the area.
Take the non permanent filt tipped pen in a dark colour like black or blue.
Draw a line from one end of the tape to the other, go over the line to build up the ink.
Now just go about your daily life. Keep an eye on the masking tape strips.
If a drip runs over it, the water will pick the ink up and spread it about on the tape.
Even if the drip is tiny and runs very slowly across the tape, the ink will still spread.
Here you can see the clear trail leading to the tape, the disturbed ink and the blue trail
of water running away from the tape. Amazingly, this was only 1 single drip of water.
This is very similar to when you used to do chromatography of ink in science.
If the tape was long enough you would get the constituants of the ink deposited at different levels down the tape. Luckily enough we have left school now and this is a car and not a science lesson.
You have located where the water is dripping down from so now you can remedy the problem.
A leaking windscreen should be taken back to the fitter.
Leaking side rear windows should be removed and checked, see here.
A leaking rear boot hatch will either need the alignment looking at, the seals adjusting or perhaps the rear window squirter pipe has come off inside the rear spoiler. See here to fix it.
If you have a leaking Roadster then take a look here and here.