I believe it depends on which stereo you buy...
I put in a 2 din Pioneer (high end but old) unit I found on ebay and after searching wiring diagrams with the plan of soldering it all in situ, I found there was a myriad of harnesses and adapters available - I just searched 'pioneer toyota wiring harness'. About £15, I think.
BUT... fitting it was a much bigger job than I'd planned. You have to take out the instrument panel, and there's not much room behind the head unit for cables. Took me half a day!