That'll throw off his impedance and make the head unit (or amp) unhappy, though. Splicing into the existing door speaker wiring would effectively halve the impedance on those channels, taking things down to 2 ohms from 4. Whatever he's using as a power source would be working twice as hard, and most sources aren't 2 ohm stable. Might work for a while, might fry some stuff. Need more info on his existing system to make a call one way or the other.
Just went through his post history and it appears he has a '97, which would mean 4x6s in the rear pillars.
@jmaple, what I would honestly suggest is selling the 6.5"s you have laying around and buying a decent set of 4x6s for the rear. If you absolutely have to use the 6.5"s, unhook the rear 4x6s and use the wires from those. That way your amp/head unit is still happy with a 4 ohm load on each of the four channels.