Using OBD to control add-on lights

What you're trying to do is overly complicated and you would need an actual auto electrician to implement. Without knowing the technical details of your particular vehicle, you won't get a straight answer. But I can tell you it varies between manufacturer and you have things like body control modules and multiples CAN buses to contend with.

When you say 'avoiding having to cut into the harness', you can actually splice into a wire without cutting it, you just need to strip some of the insulation off, basically wind the other wire around it, apply some solder and then insulate.

Or, you could just use a wire T tap connector. It will bite through the insulation until it touches the wire inside. Then if you need to take it off in the future, you're only left with a slice in the insulation so only need to put some electrical tape over it.

My advice is to forget the OBD, its going to be way more complicated and expensive to try and setup a system for detecting when the turn signals are active etc. (using PIDs) for something that can be achieved with a few dollars and an afternoon.