To the best of my knowledge you can drive your amp with an after market unit, though there is lingering doubt as I cannot be 100% sure. It may be worth determining the correct wire colours from the WDS and doing a test with any stereo unit with appropriate outputs, doesn't have to be car audio, a mains driven one will work for a quick test.
Are you sure this is the path you wish to follow? From my understanding, this will only marginally improve sound, the amp seems to be the weakest link for sound quality, though the speakers are not the greatest either. For serious sound quality improvement, a complete system replacement is called for.
If you're after added features, there are various adapters that tie into the CD changer wiring to provide the modern features you are likely looking for, and then you retain the OEM look. Not the greatest look IMO, but neither is a rectilinear DIN unit spliced into the curved dash. You do loose the use of any CD changer, but it sounds like you just have the wiring only.
I don't wish to tell you what to do, I only want you to be fully informed about what you do choose to do.