I'm not familiar with the 156, but on the 147 the fan for the heater is mounted below the glovebox. The intake for this is directly above it and, whilst there is a plastic duct panel to prevent water getting in, this doesn't seem to be entirely effective.
This duct - and the intake - are underneath the passenger-side screen trim/wiper panel.
When there is water coming, in, it does seem to gather in the middle of the footwells - from my own experience this isn't necessarily a good guide to where it's coming from! It just gathers there having bled through the felt underlayer.
If there is a coolant leak, you should detect this easily when the engine is warm and running as it will be depositing hot water into the footwell. Apart from that, it will be quite sticky due to the glycol in the antifreeze.
Check also the following:
1) Under the rear section of the front wheelarch liner:
Either take the front wheel off or turn onto full lock. Remove the screw at the lower rear edge of the wheelarch liner and pull the liner away (not it hooks onto a body seam, so needs a firm pull). Under there you will see an oval, rubber plug about 4" by 2". This should be properly seated in its hole, but isn't always...
If it isn't correct pull it away, clean thoroughly around it's edges and the hole, then refit making sure it's fully in. A thin (1/8", 3mm) bead of sealant around the grooved side of the seal - before refitting - will help to prevent it coming out again.
2) Front edge of door jamb:
Open the front door fully and look in towards the hinges. Near the lower hinge is a round rubber plug. Make sure this is seated/sealed as above.
NB. Both these plugs allow water to get inside the sill box-sections. There it tends to slosh about as you accelerate/brake and comes out wherever the nearest hole is! If you list the carpets there are two 2" rubber plugs in the inner sill panel - pull these to check for water in there. It usually shows up as surface wetness/condensation.
3) In each footwell - front/rear, left/right - there are a series of crescent-shaped slits. These are at the back end of each of the floor channels.
Driving through any amount of standing water this can get in through these drains (I've seen it happening - only last week when I had all the flooring up!) and, if the water's dirty, they silt up so the water can't get back out again.
Accessing them is a problem as they are under the bonded-on, black anti-vibration mat, but if you have to, it's worth the effort. The mat doesn't go right down into the channels, but partially seals them off forming a cavity.
The problem we found was a combination of faults:
a) The wheelarch plug not being properly fitted, allowing water to get in;
b) The door seal not being seated, again letting water in;
c) The floor drains getting blocked with fine silt, stopping the water from draining away.
Oh, and the heater fan's air inlet was dripping, too!