Water coming out of heater vent

[Bruce 21]

I have a 54 plater and with the heater fan on full blast directed to the windscreen there was water splashing up out of the top of the dashboard vent. It smelt a bit like stale anti freeze - any ideas where to start? I’ve had the top of the dashboard off but nothing obvious to see.

 

[vexorg]

Heater matrix would be the obvious start point. Lots of fun digging in behand the dash and centre console.