To find the problem:
1. Consult your owner's manual: This is the most crucial step. The manual will clearly identify the fuses for the turn signals and brake lights.
2. Check both fuse boxes: Ford E150s often have multiple fuse boxes – one under the hood and another inside the vehicle, usually under the dashboard. Check both.
3. Look for blown fuses: A blown fuse will usually have a broken filament visible inside the clear plastic or ceramic casing.
If replacing the fuses doesn't solve the problem, the issue is likely beyond a simple blown fuse and could be a faulty switch, wiring problem, or a problem with the bulbs themselves. You should then seek professional assistance from a mechanic.