If a 2015 Tacoma's bluetooth is not working, what could be the cause?

The most common reason that Your bluetooth will fail on the 2015 Toyota Tacoma comes from an unlikely offender; Your device. You won't believe how many drivers I see where their bluetooth device accidentally has the functionality disabled, the volume isn't sufficiently turned up, etc. If You've eliminated the simple causes, You can start looking into other solutions. It's possible that Your phone and Your vehicle have become unpaired; if You think this is a possibility, it's recommended that You go through the pairing procedure once more after first confirming vehicle deletion. If all else fails, it's possible that Your head unit's bluetooth transmitter has failed; unfortunately, this typically necessitates replacement of the entire head unit, as the module isn't typically standalone.