Taillights and Dashboard lights dont work anymore

In a 2004 Mercury Mountaineer, the fuse for the dashboard lights and taillights is typically located in the fuse box under the dashboard on the driver's side of the vehicle. The exact location of the fuse and the fuse number may vary depending on the specific model of Mountaineer you have.
To find the correct fuse, you will need to refer to the owner's manual for your vehicle. The owner's manual should have a diagram of the fuse box, which will show the location and function of each fuse. Alternatively, you can also check the label on the inside of the fuse box cover, which should also list the function of each fuse.
Once you have located the correct fuse, you can use a fuse tester or a multimeter to test the fuse to see if it is functioning properly. If the fuse is blown, you will need to replace it with a new one of the same rating. It is important to use a fuse with the correct amperage rating to avoid damaging the electrical system. If you are unsure which fuse to use, it is recommended to consult a professional mechanic or refer to the owner's manual for guidance.

You can check if the head of the light is worked well , and the wire is well connected.

Could the dashboard switch (for the lights) or wiring. Possible one of the harness connectors got unplugged. Check the trunk are and under the dashboard.

I was told that it is the rheostat, go to Advanced or O'Reilly's.

I think you will find a blown fuse,the tail and dash lights are on the same fuse.

check the fuses.

Try to find the relay underneath the dashboard.

you have a bad headlight switch or a bad conection at the headlight switch

check for bloown fuse