They told me the entire squib needed to be replaced. It is connected to the steering wheel inside the dash. They say it is "open" meaning it shorted out.
Some details for you: B1801 (not D) is open in driver's side squib circuit somewhere. Open means disconnected, not shorted. The circuit in question contains the spiral cable, the spiral cable connector, the instrument panel wire that connects this to the center airbag assembly and it's connectors, the center airbag sensor assembly itself, and the driver's side squib. The service procedure is to test the first four of those things in turn, and if they test okay, then it's the squib, meaning they replace the whole darned steering pad, which is expensive.
If they told you it was the squib, then it sounds like they did the cascade of testing. But if they just pulled the code, they're just guessing, and it could be in one of the first, cheaper things, and you can actually test those yourself. And repair them. For details, try googing the Toyota Tacoma FSM and check out the B1801 section. You'll at least learn a lot.
Meanwhile, the open circuit means that airbag won't deploy, so drive carefully, and think about getting it fixed sometime soon for your safety.