Look at the manual that came with the dock. it shows the cable and charger you need.
Gating that cable has proven difficult so far. I hear the Droid3 and Motorola branded dock come with it.
Ha. I guess I should read the instructions.
I actually saw this post last night and searched around a bit on the tpoic on my tablet and I concur, it appears it is difficult to get just the cable. Perhaps not even currently possible. And it does indeed appear to only come with the Motorola branded versions of the car dock. Which sucks since they are putting MSRP at $60. Amazon had it for $55. Still pricey compared to the $30 I paid for each of mine using the 25% off deal.
Regardless, I'm not sure the cable is ideal anyways. I would have preferred they stick a male 3.5mm jack on the end of that. The way they have it, you need to use an intermediate cable or a male to male adapter. Just feels like it could create an unnecessary bundle of wires near the radio. Still, I'd love to get a hold on one just to see how they did wire it.
I also noticed HTC had something similar available for some of their phones. It has me wondering if this might work on the Incredible. If so, I might ditch BT on my cars and just use the Aux-in. It would certainly eliminate a few problems we have had. Namely, I was following my girlfriend from her parents on night and she couldn't get her phone to connect to the car she was driving. Meanwhile, Pandora kept mysteriously pausing and unpausing on my phone. We didn't realize my phone was connected to both cars until we got home and she told me she was having issues connecting.