Can you link to the specific device you purchased please?
Was this just a remote switch?
Generally yes, all the 85's go to battery ground (earth) -or connect to another wire in the ground distribution. Do not use the frame as a grounding point for ANY accessories.
See the diagram in my post 33 or the one in post #38 - that is my design but elegantly re-drawn by bagman.
The two diodes - one on the wire coming back from the kill switch and the one between the remote output and the primary relay - are
important and should be installed.
The diodes across each of the relay coils can eliminated if you just buy diode or resistor suppressed relays at the outset.
Note that if eliminating the Key-Switch completely, you do not need a third relay between the red/blue and red/orange wires (blue & red on the key-switch harness) - these can just be joined together.
So you can eliminate completely. (the park light circuit is already switched by the main lighting pole - it is only required to be separate pole when the park position on the keyswitch is utilized - no keyswitch, no special park position
)
If you want to to keep the ignition circuit isolated from the ignition circuit, use that third relay instead to switch the green wire (yellow on keyswitch harness) on the 87, with a jumper from the White Blue over to the 30 of this relay. Or just leave the green and the blue/yellow tied together if you want to get down to 2 relays.