1. Consult your owner's manual: This is the best resource. The manual will have a fuse and relay diagram showing the exact location.
2. Check online forums and resources specific to your Thunderbird's year and model: Search for "[Your Thunderbird Year] fuel pump relay location" on sites like Ford forums, or general automotive forums. Many people have posted photos and detailed descriptions.
3. Look in common relay locations: Relays are often found in the under-hood fuse box (usually near the battery), or sometimes in a separate fuse box inside the passenger compartment. The under-hood fuse box will likely have a diagram on the inside of its cover.
Without knowing the year of your Thunderbird, it's impossible to give a precise location.