that audioshop is semi-right.... but the LC is not so proprietary that you can't get around it and keep your stock amp.
@lx200inAR is correct in that you're changing out speakers with higher impedance (ohms)...most aftermarket speakers are 4ohm. The cruiser's factor speakers are ~2ohm so you have to find aftermarket 6x9s for those front doors that are the same load. These do exist, you just have to search specifically for them - I know CDT makes them (i own a pair) - i would also look in Focal integration for toyota. Other posts about there being a frequency filter are also correct - if you use your existing wiring, your front 6x9s will only play midbass. the tweeters in the A pillar and 4 inch speakers in the dash will all handle the highs. Rear doors are full range, even though factory includes a separate tweeter.