It could be your a/c condensate drain, it’s on that side .
On the firewall under hood on passenger side is where this exits the cab . It’s a 90* rubber hose that is crimped on the end , this normally drains with ac on , sometimes the crimp gets crusty and causes it to backup and overflow in the cab.
This is where I would look first , next I would think maybe around the windshield . This would need to have water sprayed at every angle you can manage , as when at speed the water may run up from the bottom and cause leaks
Could be the heater core too , but it would smell like antifreeze.
Hopefully this gives you an idea where to start searching .