Wouldn't recommend earthing to the chassis for your negative. Asking for issues lanter on.
You are already splicing the positive for the Parker... Why not just splice the Negative for the Parker too and do it properly. Lol
Spend 5 mins longer now and potentially (little pun for Ryan
) save yourself pulling them out later. Earthing to the body would be ok. But for small CSA conductors with a tiny contact area to the body. I don't like doing it. Likely to break... And turns into an absolutely rubbish earth after 12months lol.
So yeh, just wire it up properly and hae a neat and tidy harness.