I bought a cable to control my 706 from K1NU. His web page is at www.k1nu.com
. Unfortunately he is on vacation until mid-August.
You just need to be sure hamlib supports turning RTS on as that is what powers the interface.
You can get a similar cable from RT Systems at http://www.rtsars.com
. The cable you need is the CT-29A. I have one for my R10 and it works on the 706 as well.