Fluid leak inside car

Probably debris caught in the cowl allowed to sit over time = rust holes.

Your gonna have to pull the heater box or keep it out of the rain/be careful when you clean it....try to clean up the debris in the cowl as much as possible. You would have to remove the air box to get at it.

Drives side is easy...3 nuts and out it comes.
Passenger side is the whole box....you have to take it out anyway to change the heater core.

Get a box rebuild kit from Detroit Muscle Tech while your at it..

Once out you may be able to seam seal it till you get around to body work.

Good luck.

Mop