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The GND in the schematic is internal, the only thing connected to anything is the 4 posts on the left hand side... Connecting to vehicle ground wouldn't really harm anything unless that is connected to pack ground... Otherwise it should be the same as negative on the 12V system...
The enable pins, two this time is +12V and ground in the 12V system...
R7 is a bypass... If you use the remote on feature you leave R7 out of the circuit, if you use it as a standalone, you stick a wire there and leave out R6 and the opto... It's an easy way to have one PCB for two functions...
The opto is about ~$0.5 in batches of more than 10 from my local supplier, so not too expensive... Dunno what you would pay/get... But it should be more than possible to find an equivalent cheap one...
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