I've replaced the doors 5 inches spk with a pair of two way 6 1/2 from sony. Great improvment!
you need to make a wooden ring to fit it properly and to give it more space for the back of the cone. (the stronger you fit, the better bass and detail you'll have)
Then I put two more (6 1/2 also) in the midle of the rear cabin near to the side door.
They work very will with the sony head unit (no need of amps or eq!)
You can use any other goodbrand, but dont put anything smaller than 6 inches spk