Aftermarket A/C with Factory Heater

[jojo]

If this has been addressed in another thread please let me know.

I've read complaints about guys not being able to defrost their window with aftermarket a/c.

I wanted to know this is possible (or even if anybody thinks its necessary in hot texas):

Can I install aftermarket A/C with factory heater core in order to retain a proper defrosting capabilities?

My concern is not only the cold but rather when it rains, is it needed to have the defroster?

Looking for anybody's input, since I'm hoping to pull the trigger on purchasing the system in the next few weeks..