HF earthing - the fun begins

I'm trying to setup a new HF (Barrett 2050 + 910 autotune Outback pack) and have a few earthing questions.

Can I have an RF earth going to the same point as my 12v -ve earth?

Basically I've run a 20mm^2 cable from the (aux) battery 12v +ve down the chassis rail to a fuse box in the back right panel of the Patrol (where the jack would normally be). I've created a -ve earth point by scraping off paint around the body. Is it ok for me to use that same negative earth point also as the RF earth for the radio?

I've earthed the 910 autotune from its mount on the rear wheel carrier to the chassis. Should I be running a braided earth cable between the antenna earth (chassis) and the radio RF earth (body)?

And where is a good place to get braided cable (Brisbane - bayside). Or can I just get some coax and butcher it?

I got a little impatient earlier on and plugged it all together using my current earth points (although I ran it using the supplied power cable directly to the battery since my 20mm^2 cable isn't yet live). I tried to listen in on the 8pm (Brisbane time) vks737 sked on ch2 but couldn't really make anything out. I could pickup the occasional word, but mostly it was static. I could get one of the Radio Aus (?) stations fairly clearly. I'm hoping my problem is earth related, since I really would rather be able to use vks737 than radio aus.

Anyway, any pointers would be appreciated.

Less than three weeks to go - Yikes!

-geoff