I would 2nd that idea of making sure you get metal contact. Just throwing a piece of bonding on painted parts will not effectively do anything, except maybe keep the bonded part from falling off the vehicle.
I had my 857 mounted near the center console. I could easily rest my arm on the top of the center console and tune around. It was comfortable. But I have had lots of experience listening with my father at home when I was younger. I have listened to poor radios with poor antennas and great radios with great antennas and also awesome radios with awesome antennas. I must say, with a beam antenna at 100' pointed anywhere with a well tuned radio and 1500W PEP AMP on your side, it is a nice warm feeling. You only get that with a less equipped station when you make that QSO that should not have been made. A 5-9+20 signal report from Spain at 4:30 in the afternoon traveling through Denver rush hour traffic gets me that warm feeling. Especially barefoot mobile.