You don't have to buy an adapter to do the bypass.
You have a few options:
You can install the deck and run it into the amp. This will most likely result in extreme distortion at anything above a moderate volume level.
You can run alternate speaker wires into the doors from the HU, but I believe many have found this to be a pain in the backside.
If you really want your aftermarket deck to sound as good as it can, then I would recommend buying the adapter. Crutchfield sells one on their site, and provide it free with any HU purchase. Or, you can do what I did -- I bought a Metra kit, and soldered a few wires together to make a bypass cord of my own. It only ran me about C$15 for the parts, plus some time to put it all together.
Good luck.
Brian
