When I put my plug in my RRS I also researched a lot on here and got varying answers.  Most said go back to the battery (earth point) but then I thought what is the difference in earthing locally at the rear?  Many posters suggested that that you could not guarantee earth ultimately back to the battery but in a welded car this should not be an issue.  Now the battery is earthed to the body and not to the chassis (there are various earth traps form the body to the chassis/engine/gerabox etc) so, though unlikely if you earthed the anderson to the chassis the above issue could arise though unlikely.
Therefore I reasoned earthing to the body at the rear was basically the same as taking a earth wire all the way back to near the battery earth point - remember the cars body is an earth for most functions anyway. 
I just drilled a small hole in the boot outer floor just above the spare, cleaned it up for a good connection bolted the Anderson earth through the hole and then covered the lot with sealant inside and out(seam sealer in my case).
I have not had any issues with the connection or powering equipment from the plug.
Just make sure you earth to the body and not the chassis though really there shouldn't be an issue with the latter.
Garry
				
			 
			
		 
			
				
			
			
				REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101 
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
			
			
		 
	
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