the best ground is either the engine block or the battery. On one car, I drilled a hole thru the firewall and installed a bolt. I hooked the ground up to this bolt under the dash and under the hood, I connected a ground strap to the engine from the same bolt.
The ign terminal should work ok for power. On my AFX, I think I connected power to a switched ignition source under the hood and the ground to the battery ground where it connects to the inner fender.