Pull the radio and unplug the antenna lead from the back! Remove two screws at top of air vents on the center trim piece - pop off the trim piece. Remove the four 10mm screws that secure the head unit (two on each side) and pull it out. There are two antenna wires. The large one is a standard size, the smaller one is the diversity antenna.
Keep in mind that there is a factory signal amplifier on the standard antenna wire, so if you intend to put an aftermarket head unit in, you will want to maintain the power connection to the signal amplifier, or you will get very poor or no FM reception.
Well I'm new to the forums, but I think to change antennas you have to remove the right front fender well cover. To do this you will have to remove some plastic fasteners and it is recommended that you buy the number you take out from a toyota dealership. Since when you reinstall the fender well cover you will need new fasteners. Once you get the fenderwell cover removed (I recommend you remove the right front tire before you remove the right front fender well cover ), you should be able to get to the antenna to remove it. Also, I read on this forum that if you wanted to install a power antenna in a Tundra, your best bet is to get a used Sequoia power antenna. Take care.