I have a 2002 Park Avenue with the base level sound system. I have a monoblock amp and twin 12" subwoofers from another car that I'd like to put into the Buick. What do I need to wire up the system and keep the stock radio. Any tips on doing this and keeping the stock look?

You need a line level converter. IMO, your wasting time unless you upgrade the front speakers. If your going to upgrade those, you might as well upgrade the HU. Its a general rule of thumb, upgrade all 3 within your budget. The 12'* will over power your front speakers and everything will sound muddy.