Welcome to TN and like Zrain said, to The 5th Gen club.
If your lights dimm down when pressing the brakes or when the blinkers are on the problem are;
1.- Voltage regulartor, housed inside the alternator, so yes, you have to take it all out. Just get to whole thing serviced if needed.
2.- Alternator itself. You can get it serviced, Celica alternators are not like most other alternators that are sealed and non serviceble... you can get parts needed at your local Toy dealer (or at least here I can)
And remember... a Celica is not a Celica unless it has pop up lights