Does this need any wiring modifications to install?

It plugs into the existing factory connector — no rewiring necessary. If your Land Cruiser still has the original third brake light wiring intact, this is a straight swap.

Will this fit if I've already removed the factory third brake light?

As long as the mounting bracket and wiring harness are present, yes. If either has been deleted or disconnected, you'll need to restore the connection before installation.

What's the lifespan of the LED versus a halogen bulb?

LEDs outlast halogen by thousands of hours, so you won't be replacing this bulb anytime soon. The assembly is sealed, so you're buying durability and consistency over the life of your truck.