"Hey guys, I just did a headunit install on my G/F's 2000 Celica GTS. The external amplifier is under the dash on the right side of the glove box. Pull the glove box, and you will see it. There are two harnesses that plug into it. Pull them both, and disregard the one with the thick grey wire like the one up at your headunit. If you go to a stereo store, they give you an adapter harness that has 2 different plugs, one small, one a little larger. If you take the plug you removed from the amp and look, you will see that it is one piece, and has two indentation marks on it on the top, one on each side. If you take the larger aftermarket adapter and hold it up between those two indentations, you will notice that it would plug in provided the sides were cut off... If you do that tho, beware, the wire colors are not the same as the radio, and it will not hook up your front door tweeters. Use the info that the guy posted above and hard wire it in. I made a custom harness w/ 4 sets of speaker wire 2.5 feet long, a yellow power, red memory, and I grounded it out at the support bar directly behind the radio. This works out great! I cut off the stock plugs and hard wired the radio in, but if you were considering selling the car again someday, you could use T-Taps to tap into the wires, that way you retain the stock plug. If anyone has any car audio how to questions, let me know! "
quoed from here https://www.celicahobby.com/ubbthreads/thread.f_57398_0_collapsed_5__1.html#57398
Hope it helps