I use a similar approach:
I bought the HTC adapter here for $6.79, which is very versatile.
http://www.amazon.com/Mini-USB-Heads.../dp/B002HNTHKM
It allows me to keep the phone charged with a car adapter/USB connector. I then plug this into the 3.5mm port. Best Buy had it for $6.99:
http://www.google.com/products/catal...wAg#ps-sellers
My cassette adapter plugs into this (you can also use aux in if you have that - my car doesn't), and I route the microphone up the driver's seat pillar where I have easy access to the button which activates Voice Command, and picks up my voice fairly well. In my case, the cable wasn't quite long enough, so I picked this up from KV Connect for $8.18:
http://www.kvconnection.com/product-p/km-24-10461.htm
So for about $20, I have the functional equivalent of a built-in system to take calls, as it would work if my car had built-in Bluetooth, which it doesn't. I mount my TP2 on the vent near the driver's side, and can see the display for phone information, incoming texts, GPS when I use it, etc. A little "cludgy" but it works pretty well. If the Audio volume is turned up too high, the caller will hear some echo, but at reasonable and easily hearable levels, it works just fine.
Just in case anybody is looking for a similar setup - it was actually pretty hard to find those cables and connectors.