At first, I spliced directly into the wire under the carpet. However, I ended up undoing that and plugging my dashcam into the OBD2 port directly for two reasons.
First, people that know more about this stuff than I do told me that I shouldn't use those cheap snap together wire splices. I wasn't a big fan of them either because it was very hard to get them to clamp down all the way and stay closed. After a couple of weeks, I looked at the splice, and it was starting to open up.
Second, I preferred the idea of plugging into the OBD2 port instead of encouraging people to splice into wires in the car. I think it's fine for a professional installer to splice into wires because they (in theory) know what they're doing and what wires they should splice into and which ones they should leave alone.