Leaky LV

[TripleJ85]

My wife drives a 2010 LV II LX Manual that we've had from new. On the whole, it has been a great little commuter car and still has plenty of life left in it (hasn't hit 100k km yet).

About 3 years back the driver's footwell got saturated and the car got very stinky.
A local mechanic had a look and found that the gutter under the windscreen wipers was full of schmutz and the drain was blocked - he cleared out the drain hose but didn't find anything else wrong. This mostly fixed it - in that it could handle moderate amounts of rain or hand washing... but being out in heavy rain or automated carwashing would make it a bit wet and the stink would return. Not a huge amount of water though so fairly easy to dry out again.
It has mostly been parked inside and we haven't had a lot of rain since then so had let it be.
We took it to an autoglass place who hit it with a firehose and couldn't see any leaks (but the next day it got stinky and wet under the carpet).

We've had a few storms lately so it is back on the to-fix list...

It was in for a service last week and the mechanic tried a smoke test - but couldn't find anything. He suggested the next step would be to pull the dash out to try to find the leak (but found it hard to recommend this on a car that's not worth much).

So I thought I'd put some feelers out here - from what I can gather on other forums, it's fairly common to have a leak somewhere on the bulkhead. Do you guys have any tips on how to find where it might be? Or who would be best to help us find it (panel beaters?). Has anyone else been down the same path?

It would be a shame to give up on the car over something this minor - but it is really driving her nuts!