The parking break lead has a wiretap on the end--can i tap into the ground from the pioneer wireharness to the aftermarket wireharness and ground the parking break that way (using the wiretap)?

yes

dspiel is right. You can definitely do that. If the factory harness has a ground wire, then it's usually perfectly fine to tap into that cable.

In fact, that's what I did.

ditto.. i clamped both the P brake and mute wire on my D3 into the factory harness ground. works beautifully

Doh I forgot to ground the mute wire--- So I can tap the ground wire with both the mute wire and parking brake wire, right?