2008 Ford Mustang - The passenger side under the dashboard recently leaked

I took it to a Firestone recently and they said I had to take it to the dealer or to Ford directly. Called Ford and they said it would be $130 just to even look at it. The dealership I bought the car from is closed for good now. Can anyone tell me if there's a way to fix this yourself rather than having it fixed? The passenger side leaks everytime it rains but it rained heavy the other day and now the carpet is flooded.

There's a few possible reasons for the leakage, so unless you know someone who will take the time to look and try to find the source, you will need to have the dealer or an independent garage try to fix it

Usually it is the heater core or lines leading to the heater core that leak onto the passenger side floor. Get some dye and make some colored water. Pour it over the windshield, hood and roof near the passenger seat.

Ford dealer will likely be best quick fix though not cheap. They may have a record of where leaks originate in that model and will have ability and spares to fix.
many models will have manufactures fault finding chart including leaks available to main dealers plus of course how to fix! Independents not so.Apart from obvious check such as is coolant level dropping when we get leak? But as floor east wet after heavy rain an you have not commented don coolant level then likely hood is external leak and not engine issue/heater pipe etc.
in a few cases we may? Be able to see for self by looking in car with doors shut and using a hose to ask from roof and screen. Its possible we may then see an are of leak but beware water is well known for running along seams etc and puddling in an area that isn't giving us actual source of issue.One car for example had water on well floor from hood/screen vents as drain was blocked onto road and as level rose it found a way between seam into car.