when I turn on the car I can hear like some type of liquid moving around sounds like from behind the glove box. anyone have any idea as to what it could be?
Hello; two notions. First is the drain for the AC condensate is plugged up. Not sure on a mustang but usually a tube which exits under the car. Get the car up high enough so you can safely be under the car. Poke gently into the hose with something to loosen up the junk.
The other notion is the area in front of the windshield where the wiper motor is located. I call it a cowl. This area is sort of like a tray. The area also usually feeds air into the vents- heater-AC box. I have seen leaves and other debris clog up the drain for this area. Drain likely to be similar to the AC drain (Might be one drain for both) Same sort of fix, which is to clear debris.